batman-adv: Avoid redundant multicast TT entries

If a node signals that it wants all traffic for a specific protocol
family then there is no need to announce individual multicast addresses
via TT.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>