The VEB (virtual embedded switch) as a switch element can be
connected according datasheet though its uplink to:
- Physical port
- Port Virtualizer (not used directly by i40e driver but can
  be present in MFP mode where the physical port is shared
  between PFs)
- No uplink (aka floating VEB)

But VEB uplink cannot be connected to another VEB and any attempt
to do so results in:

"i40e 0000:02:00.0: couldn't add VEB, err -EIO aq_err I40E_AQ_RC_ENOENT"

that indicates "the uplink SEID does not point to valid element".

Remove this logic from the driver code this way:

1) For debugfs only allow to build floating VEB (uplink_seid == 0)
   or main VEB (uplink_seid == mac_seid)
2) Do not recurse in i40e_veb_link_event() as no VEB cannot have
   sub-VEBs
3) Ditto for i40e_veb_rebuild() + simplify the function as we know
   that the VEB for rebuild can be only the main LAN VEB or some
   of the floating VEBs
4) In i40e_rebuild() there is no need to check veb->uplink_seid
   as the possible ones are 0 and MAC SEID
5) In i40e_vsi_release() do not take into account VEBs whose
   uplink is another VEB as this is not possible
6) Remove veb_idx field from i40e_veb as a VEB cannot have
   sub-VEBs

Tested using i40e debugfs interface:
1) Initial state
[root@cnb-03 net-next]# CMD="/sys/kernel/debug/i40e/0000:02:00.0/command"
[root@cnb-03 net-next]# echo dump switch > $CMD
[root@cnb-03 net-next]# dmesg -c
[   98.440641] i40e 0000:02:00.0: header: 3 reported 3 total
[   98.446053] i40e 0000:02:00.0: type=19 seid=392 uplink=160 downlink=16
[   98.452593] i40e 0000:02:00.0: type=17 seid=160 uplink=2 downlink=0
[   98.458856] i40e 0000:02:00.0: type=19 seid=390 uplink=160 downlink=16

2) Add floating VEB
[root@cnb-03 net-next]# echo add relay > $CMD
[root@cnb-03 net-next]# dmesg -c
[  122.745630] i40e 0000:02:00.0: added relay 162
[root@cnb-03 net-next]# echo dump switch > $CMD
[root@cnb-03 net-next]# dmesg -c
[  136.650049] i40e 0000:02:00.0: header: 4 reported 4 total
[  136.655466] i40e 0000:02:00.0: type=19 seid=392 uplink=160 downlink=16
[  136.661994] i40e 0000:02:00.0: type=17 seid=160 uplink=2 downlink=0
[  136.668264] i40e 0000:02:00.0: type=19 seid=390 uplink=160 downlink=16
[  136.674787] i40e 0000:02:00.0: type=17 seid=162 uplink=0 downlink=0

3) Add VMDQ2 VSI to this new VEB
[root@cnb-03 net-next]# dmesg -c
[  168.351763] i40e 0000:02:00.0: added VSI 394 to relay 162
[  168.374652] enp2s0f0np0v0: NIC Link is Up, 40 Gbps Full Duplex, Flow Control: None
[root@cnb-03 net-next]# echo dump switch > $CMD
[root@cnb-03 net-next]# dmesg -c
[  195.683204] i40e 0000:02:00.0: header: 5 reported 5 total
[  195.688611] i40e 0000:02:00.0: type=19 seid=394 uplink=162 downlink=16
[  195.695143] i40e 0000:02:00.0: type=17 seid=162 uplink=0 downlink=0
[  195.701410] i40e 0000:02:00.0: type=19 seid=392 uplink=160 downlink=16
[  195.707935] i40e 0000:02:00.0: type=17 seid=160 uplink=2 downlink=0
[  195.714201] i40e 0000:02:00.0: type=19 seid=390 uplink=160 downlink=16

4) Try to delete the VEB
[root@cnb-03 net-next]# echo del relay 162 > $CMD
[root@cnb-03 net-next]# dmesg -c
[  239.260901] i40e 0000:02:00.0: deleting relay 162
[  239.265621] i40e 0000:02:00.0: can't remove VEB 162 with 1 VSIs left

5) Do PF reset and check switch status after rebuild
[root@cnb-03 net-next]# echo pfr > $CMD
[root@cnb-03 net-next]# echo dump switch > $CMD
[root@cnb-03 net-next]# dmesg -c
...
[  272.333655] i40e 0000:02:00.0: header: 5 reported 5 total
[  272.339066] i40e 0000:02:00.0: type=19 seid=394 uplink=162 downlink=16
[  272.345599] i40e 0000:02:00.0: type=17 seid=162 uplink=0 downlink=0
[  272.351862] i40e 0000:02:00.0: type=19 seid=392 uplink=160 downlink=16
[  272.358387] i40e 0000:02:00.0: type=17 seid=160 uplink=2 downlink=0
[  272.364654] i40e 0000:02:00.0: type=19 seid=390 uplink=160 downlink=16

6) Delete VSI and delete VEB
[  297.199116] i40e 0000:02:00.0: deleting VSI 394
[  299.807580] i40e 0000:02:00.0: deleting relay 162
[  309.767905] i40e 0000:02:00.0: header: 3 reported 3 total
[  309.773318] i40e 0000:02:00.0: type=19 seid=392 uplink=160 downlink=16
[  309.779845] i40e 0000:02:00.0: type=17 seid=160 uplink=2 downlink=0
[  309.786111] i40e 0000:02:00.0: type=19 seid=390 uplink=160 downlink=16
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>