backlight: do nothing if max_brightness is 0

backlight: do nothing if max_brightness is 0

On virtually any newer Asus mainboard, the eeepc-wmi driver is loaded.
It exposes a backlight device despite the lack of any physical backlight
devices. This fake backlight device has max_brightness set to 0. Since
the introduction of the clamp_brightness function, systemd-backlight
tries to write '1' to brightness and fails.

This patch changes systemd-backlight to exit gracefully when
max_brightness is 0 before performing any action. This affects
both the load and save actions.