backlight: panasonic-laptop - 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>
Acked-by: Harald Welte <laforge@gnumonks.org> (registration failure)
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>