rtlwifi: Assign rx buffer ownership to hardware last

Ownership of an rx buffer should only be given to the hardware
after all other changes are written, otherwise there's
a potential race.

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>