asus-wmi: provide access to ALS control

Asus Zenbook ux31a is providing ACPI0008 interface for ALS
(Ambient Light Sensor), which is accessible for OS => Win 7.
This sensor can be used with iio/acpi-als driver.
Since it is disabled by default, we should use asus-wmi
interface to enable it.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>