backlight: lm3639: Use devm_backlight_device_register()

Change to use devm_backlight_device_register() for simple cleanup.

Signed-off-by: Daniel Jeong <gshark.jeong@gmail.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>