HID: asus: only support backlight when it's not driven by WMI

The Asus GL502VSK has the same 0B05:1837 keyboard as we've seen in
several Republic of Gamers laptops.

However, in this model, the keybard backlight control exposed by hid-asus
has no effect on the keyboard backlight. Instead, the keyboard
backlight is correctly driven by asus-wmi.

With two keyboard backlight devices available (and only the acer-wmi
one working), GNOME is picking the wrong one to drive in the UI.

Avoid this problem by not creating the backlight interface when we
detect a WMI-driven keyboard backlight.

We have also tested Asus GL702VMK which does have the hid-asus
backlight present, and it still works fine with this patch (WMI method
call returns UNSUPPORTED_METHOD).

A direct "depends on ASUS_WMI" is intentionally avoided so that HID_ASUS
users who have ASUS_WMI=n will not quietly lose their HID_ASUS driver on
a kernel upgrade.

Signed-off-by: Daniel Drake <drake@endlessm.com>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>