Currently dsa_port_vid_add returns 0 if the switch returns -EOPNOTSUPP.

This function is used in the tag_8021q.c code to offload the PVID of
ports, which would simply not work if .port_vlan_add is not supported
by the underlying switch.

Do not skip -EOPNOTSUPP in dsa_port_vid_add but only when necessary,
that is to say in dsa_slave_vlan_rx_add_vid.
Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>