ath6kl: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in struct
ath6kl_vif, use stats from struct net_device. Also remove the now
unnecessary .ndo_get_stats function.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>