staging: vt6655: Use net_device_stats from struct net_device

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

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