batman-adv: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in struct
batadv_priv, use stats from struct net_device.

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