staging: gdm72xx: Use net_device_stats from struct net_device

Instead of using an own copy of struct net_device_stats in struct nic,
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().

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