orinoco: Use net_device_stats from struct net_device

Instead of using a private copy of struct net_device_stats in
struct orinoco_private, 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@codeaurora.org>