We can't have more than one default VSI so prevent another VSI from
overwriting the current dflt_vsi. This was achieved by adding the
following functions:

ice_is_dflt_vsi_in_use()
- Used to check if the default VSI is already being used.

ice_is_vsi_dflt_vsi()
- Used to check if VSI passed in is in fact the default VSI.

ice_set_dflt_vsi()
- Used to set the default VSI via a switch rule

ice_clear_dflt_vsi()
- Used to clear the default VSI via a switch rule.

Also, there was no need to introduce any locking because all mailbox
events and synchronization of switch filters for the PF happen in the
service task.
Signed-off-by: Brett Creeley <brett.creeley@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>