backlight: backlight_device_register() return ERR_PTR()

backlight_device_register() returns a valid pointer or ERR_PTR() never
NULL.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>