backlight: msi-laptop, msi-wmi: fix incomplete registration failure handling

Properly return backlight registration error to parent.
Mark struct backlight_ops as const.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Reviewed-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>