backlight: fix class_find_device() arguments

Michał's previous patch missed this backlight check to fix up the
class_find_device() arguments.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>