platform/x86: asus-wmi: Keyboard backlight brightness does not work correctly on Asus ROG Zephyrus G15

[Mitchell Augustin <mitchell@mitchellaugustin.com>]

Hello,

On my Zephyrus G15 (GA503RM-G15.R93060),
 my keyboard brightness controls do not work. I believe I have traced 
this down to being a kernel issue, based on the fact that my Gnome 
control center slider and `asusctl -k` do result in 
`/sys/class/leds/asus::kbd_backlight/brightness` being updated, but the 
changed value of `/sys/class/leds/asus::kbd_backlight/brightness` is not
 reflective of the actual brightness of my keyboard, which always 
remains unchanged. (I can also confirm that other keyboard/fan control 
features do work, such as the keyboard profile switcher, custom fan 
curves, etc. - so it just seems to be the brightness that is broken.)

If this is likely just a case where I would need to poke around my hardware to 
find the correct values to write for brightness control to work on my 
specific SKU, I would be happy to do that and contribute it back here, but I 
will need some guidance on where to look for that info.

Let me know if that would be helpful, or if there's any other info I can provide to help fix this bug.

Thanks,

Mitchell Augustin