batman-adv: constify and move broadcast addr definition

The variable is used only once and is read-only. Make it a const local
variable.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>