firewall: Allow IGMP and MLD input on WAN

The WAN port should at least respond to IGMP and MLD queries as
otherwise a snooping bridge/switch might drop traffic.

RFC4890 recommends to leave IGMP and MLD unfiltered as they are always
link-scoped anyways.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

SVN-Revision: 45613