rtlwifi: rtl8192se: Use write barrier when assigning ownership

As noted by Mike McCormack <mikem@ring3k.org> for rtl8192ce, make sure
all updates to a descriptor are flushed to memory before assigning ownship
to hardware.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>