backlight: Change the return type of backlight_update_status() to int

Backlight device returns the result of update_status(), but
backlight_update_status() ignores it. So the consumers cannot confirm the
result of their function call. This patch makes the result to be returned
back for consumers.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>