batman-adv: Do not add multicast MAC addresses to translation table

The current translation table mechanism is not suitable for multicast
addresses and we are currently flooding such frames anyway.

Therefore this patch prevents multicast MAC addresses being added to the
translation table.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Acked-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>