backlight: classmate-laptop - Fix missing registration failure handling

Check newly registered backlight_device for error and properly
return error to parent.
Mark struct backlight_ops as const.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>

diff --cc drivers/platform/x86/classmate-laptop.c
Simple merge