usbnet: convert rndis driver to use dev_get_stats

dev_get_stats() handles all issues with net_device_ops

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>