rndis_host: Fix sparse warning

rndis_unbind and usbnet_cdc_unbind don't return anything.

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