platform/x86: asus-wmi: Only map brightness codes when using asus-wmi backlight control

platform/x86: asus-wmi: Only map brightness codes when using asus-wmi backlight control

Older Asus laptops change the backlight level themselves and then send
WMI events with different codes for different backlight levels.

The asus-wmi.c code maps the entire range of codes reported on
brightness down keypresses to an internal ASUS_WMI_BRN_DOWN code:

define NOTIFY_BRNUP_MIN                0x11
define NOTIFY_BRNUP_MAX                0x1f
define NOTIFY_BRNDOWN_MIN              0x20
define NOTIFY_BRNDOWN_MAX              0x2e

        if (code >= NOTIFY_BRNUP_MIN && code <= NOTIFY_BRNUP_MAX)
                code = ASUS_WMI_BRN_UP;
        else if (code >= NOTIFY_BRNDOWN_MIN && code <= NOTIFY_BRNDOWN_MAX)
                code = ASUS_WMI_BRN_DOWN;

This mapping is causing issues on new laptop models which actually
send 0x2b events for printscreen presses and 0x2c events for
capslock presses, which get translated into spurious brightness-down
presses.

This mapping is really only necessary when asus-wmi has registered
a backlight-device for backlight control. In this case the mapping
was used to decide to filter out the keypresss since in this case
the firmware has already modified the brightness itself and instead
of reporting a keypress asus-wmi will just report the new brightness
value to userspace.

OTOH when the firmware does not adjust the brightness itself then
it seems to always report 0x2e for brightness-down presses and
0x2f for brightness up presses independent of the actual brightness
level. So in this case the mapping of the code is not necessary
and this translation actually leads to spurious brightness-down
presses being send to userspace when pressing printscreen or capslock.

Modify asus_wmi_handle_event_code() to only do the mapping
when using asus-wmi backlight control to fix the spurious
brightness-down presses.

Reported-by: James John <me@donjajo.com>
Closes: https://lore.kernel.org/platform-driver-x86/a2c441fe-457e-44cf-a146-0ecd86b037cf@donjajo.com/
Closes: https://bbs.archlinux.org/viewtopic.php?pid=2123716
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20231017090725.38163-3-hdegoede@redhat.com