mlxsw: fix SWITCHDEV_OBJ_ID_PORT_MDB

There is a missing break statement so we always return -EOPNOTSUPP.

Fixes: 3a49b4fd ('mlxsw: Adding layer 2 multicast support')
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>