staging: wlan-ng: Use net_device_stats from struct net_device

Instead of using an own copy of struct net_device_stats in struct
wlandevice, use stats from struct net_device. Also remove the thus
unnecessary .ndo_get_stats function, as it would now just return
netdev->stats, which is the default in dev_get_stats().

Furthermore, convert prefix increment of stats counters to the more
common postfix increment idiom.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>