mlxsw: spectrum: Check if port is vPort using its VID

When L3 interfaces will be introduced a vPort won't necessarily have a
FID assigned to it. This can happen if it's not member in a bridge (in
which case it's assigned a vFID) or doesn't have an IP address (in which
case it's assigned an rFID).

Therefore, instead check the VID parameter to test whether a port is a
vPort or not.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>