batman-adv: Add dummy soft-interface rx mode handler

We do not actually need to set any rx filters for the virtual batman
soft interface. However a dummy handler enables a user to set static
multicast listeners for instance.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>