batman-adv: remove unnecessary type castings

remove unnecessary void* type castings.

Signed-off-by: Yu Zhe <yuzhe@nfschina.com>
[sven@narfation.org: Fix missing const in batadv_choose* functions]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>