platform/x86: asus-wmi: Add keyboard backlight toggle support

Some ASUS laptops like UX550GE has hotkey (Fn+F7) for keyboard
backlight toggle which would emit the scan code 0xc7 each keypress.
On the UX550GE, the max keyboard brightness level is 3 so the
toggle would not be simply on/off the led but need to be cyclic.
Per ASUS spec, it should increment the brightness for each keypress,
then toggle(off) the LED when it already reached the max level.

Signed-off-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>