net: bridge: mcast: prepare add-router function for mcast router split

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

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