batman-adv: Remove needless init of variables on stack

Some variables are overwritten immediatelly in a functions. These don't
have to be initialized to a specific value on the stack because the value
will be overwritten before they will be used anywhere.

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