batman-adv: Omit unnecessary memset of netdev private data

The memory for netdev_priv is allocated using kzalloc in alloc_netdev
(or alloc_netdev_mq respectively) so there is no need to set it to 0
again.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>