staging: xillybus: Remove mmiowb() before wait_event_*() calls

According to Documentation/memory-barriers.txt, a memory barrier is put
in place by wait_event_*()

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>