net: bridge: multicast: use multicast contexts instead of bridge or port

Pass multicast context pointers to multicast functions instead of bridge/port.
This would make it easier later to switch these contexts to their per-vlan
versions. The patch is basically search and replace, no functional changes.

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