HID: asus: support backlight on USB keyboards

The latest USB keyboards shipped on several ASUS laptop models
(including ROG laptop models such as GL702VMK) have the keyboards
backlight controlled by the keyboard firmware.

The firmware implements at least 3 different commands:
- Init command (to use when the system starts)
- Configuration command (to get keyboard status/information)
- Backlight level control (to change the level of the keyboard light)

With this patch we create the usual 'asus::kbd_backlight' led class
entry to control the keyboard backlight.

[jkosina@suse.cz: remove pointless cancel_work_sync() call while
 handling an error in asus_kbd_register_leds(), as spotted by
 Benjamin]

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>