usbnet: kaweth: Use net_device_stats from struct net_device

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

Cc: linux-usb@vger.kernel.org
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>