We were changing return values and accidentally made
rocker_world_port_obj_vlan_add() into a no-op.

Fixes: fccd84d4 ('rocker: return -EOPNOTSUPP for undefined world ops')
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Acked-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>