batman-adv: Reduce preprocessor checks in multicast.c

It is not necessary to disable these code sections in case other kernel
features are disabled. Instead the IS_ENABLED tests can be added directly
in the code and the compiler can remove the unnecessary code parts during
its optimization run.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>