net: bridge: mcast: prepare expiry functions for mcast router split

In preparation for the upcoming split of multicast router state into
their IPv4 and IPv6 variants move the protocol specific timer access to
an ip4 wrapper function.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>