Read from actual_brightness instead of brightness

From https://www.kernel.org/doc/Documentation/ABI/stable/sysfs-class-backlight

What:		/sys/class/backlight/<backlight>/brightness
Date:		April 2005
KernelVersion:	2.6.12
Contact:	Richard Purdie <rpurdie@rpsys.net>
Description:
		Control the brightness for this <backlight>. Values
		are between 0 and max_brightness. This file will also
		show the brightness level stored in the driver, which
		may not be the actual brightness (see actual_brightness).
Users:		HAL

What:		/sys/class/backlight/<backlight>/actual_brightness
Date:		March 2006
KernelVersion:	2.6.17
Contact:	Richard Purdie <rpurdie@rpsys.net>
Description:
		Show the actual brightness by querying the hardware.
Users:		HAL