mlxsw: spectrum: Handle VLAN devices linking / unlinking

When a VLAN interface is configured on top of a physical port we should
associate the VLAN device with the matching vPort. Likewise, when it's
removed, we should revert back to the underlying port netdev.

While not a must, this is consistent with port netdevs and also provides
a more accurate error printing via netdev_err() and friends.

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>