gluon-ebtables-limit-arp: do not add multicast addresses to filter

With the batman-adv multicast support compiled back in again we end up
with multicast addresses in the batman-adv translation table.

Currently we wrongly interpret multicast addresses returned by TT as a
unique host, too, which adds them with a source address filter to
ebtables as well. However, the source address of an ethernet frames is
never supposed to be a multicat one.

This leads to unnecessary entries in ebtables. Fixing this by ignoring
those MAC addreses returned by TT which have the multicast bit set.

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