USB: appledisplay: fix race between reading and writing from the device

The workqueue handler may call appledisplay_bl_get_brightness() while
user space calls appledisplay_bl_update_status(). As they share a
buffer that must not happen. Use a mutex for mutual exclusion.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>