usbnet drivers: From *_uno_init, call *_uno_stop, not usbnet_stop.

Make usbnet_stop private now that no drivers use it.

None of the driver-independent logic in usbnet_stop has any effect at
this point because we are guaranteed not to be running, so only the
driver-dependent logic in *_uno_stop (at most) is needed.

For drivers with no *_uno_stop, just omit the call to usbnet_stop
altogether.

Some of this logic is obviously redundant with the subsequent call to
*_reset -- to be addressed in a subsequent commit.

(riastradh)