usbnet: support net_device_ops

Use net_device_ops for usbnet device, and export for use
by other derived drivers.

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>