backlight: core: Replace kfree with put_device

As per the comments on device_register, we shouldn't call kfree()
right after a device_register() failure. Instead call put_device(),
which in turn will call bl_device_release resulting in a kfree to the
full structure.

Signed-off-by: Levente Kurusa <levex@linux.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>