USB: appledisplay: close race between probe and completion handler

USB: appledisplay: close race between probe and completion handler

There is a small window during probing when IO is running
but the backlight is not registered. Processing events
during that time will crash. The completion handler
needs to check for a backlight before scheduling work.

The bug is as old as the driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240912123317.1026049-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>