platform/x86: asus-wmi: Add lightbar led support

Some Asus laptops (ROG series for example) are provided with a lightbar
behind the monitor. This patch make possible to switch it on and off.
This lightbar works exactly like any other led.

Signed-off-by: Maxime Bellengé <maxime.bellenge@gmail.com>
[dvhart: fix commit message format and variable declaration order]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>