rtlwifi: Use write barrier when assigning ownership

Make sure all updates to a descriptor are flushed to memory
before assigning ownship to hardware.

Signed-off-by: Mike McCormack <mikem@ring3k.org>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>