myri10ge: Use the instance of net_device_stats from net_device.

Since net_device has an instance of net_device_stats,
we can remove the instance of this from the private myri10ge_priv structure.

Signed-off-by: Ajit Khaparde <ajitk@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>