switchdev: bridge: Offload mc router ports

Offload the mc router ports list, whenever it is being changed.
It is done because in some cases mc packets needs to be flooded to all
the ports in this list.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>