asus-nb-wmi: ignore ALS notification key code

Ignore Ambient Light Sensor notification key code 0xC6 found in ASUS UX31a.
When the illuminance changes dmesg shows:
[10814.939979] asus_wmi: Unknown key c6 pressed

Signed-off-by: Viktar Vauchkevich <victorenator@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>