platform/x86: asus-wmi: add keyboard brightness event handler

platform/x86: asus-wmi: add keyboard brightness event handler

The keyboard brightness control of Asus WMI keyboards is handled in
kernel, which leads to the shortcut going from brightness 0, to 1,
to 2, and 3.

However, for HID keyboards it is exposed as a key and handled by the
user's desktop environment. For the toggle button, this means that
brightness control becomes on/off. In addition, in the absence of a
DE, the keyboard brightness does not work.

Therefore, expose an event handler for the keyboard brightness control
which can then be used by hid-asus. Since this handler is called from
an interrupt context, defer the actual work to a workqueue.

In the process, introduce ASUS_EV_MAX_BRIGHTNESS to hold the constant
for maximum brightness since it is shared between hid-asus/asus-wmi.

Reviewed-by: Luke D. Jones <luke@ljones.dev>
Tested-by: Luke D. Jones <luke@ljones.dev>
Acked-by: Benjamin Tissoires <bentiss@kernel.org>
Reviewed-by: Denis Benato <benato.denis96@gmail.com>
Signed-off-by: Antheas Kapenekakis <lkml@antheas.dev>
Link: https://patch.msgid.link/20260122075044.5070-11-lkml@antheas.dev
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>