net: bridge: mcast: br_multicast_set_port_router takes multicast context as argument

Change br_multicast_set_port_router to take port multicast context as
its first argument so we can later use it to control port/vlan mcast
router option.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>