platform/x86: asus-wmi: Call led hw_changed API on kbd brightness change

Make asus-wmi notify on hotkey kbd brightness changes, listen for
brightness events and update the brightness directly in the driver.
Create new do_kbd_led_set function for in-driver update, and leave
kbd_led_set for original led_classdev call path.

Update the brightness by led_classdev_notify_brightness_hw_changed.
This will allow userspace to monitor (poll) for brightness changes
on the LED without reporting via input keymapping.

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