staging: et131x: Use net_device_stats from struct net_device

Instead of using an own copy of struct net_device_stats in struct
et131x_adapter, use stats from struct net_device.

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