backlight: blackfin - 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>
Acked-by: Mike Frysinger <vapier@gentoo.org> (constify struct backlight_ops)
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>