rocker: fix rocker_world_port_obj_vlan_add()

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: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>