asus-wmi: Add ambient light sensor toggle key

Ausu laptops issue key 0x7A when the toggle ALS key is pressed (Fn+A on
Asus U38N). Update the key_entry so userspace can handle the event.

Tested on Asus U38N.

Signed-off-by: Nick Leiten <nickleiten@gmail.com>
Reviewed-By: Corentin Chary <corentin.chary@gmail.com>
[dvhart: cleaned up commit message and comment line length]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>