usbnet: Add stop function pointer to 'struct rndis_data'.

Allow minidriver to know that netdev has stopped. This is to let
wireless turn off radio when usbnet dev is stopped.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>