batman-adv: Remove "default n" in Kconfig

The "default n" is the default value for any bool or tristate Kconfig
setting. It is therefore not necessary to add it to a config entry.

Reported-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>