platform/x86: asus-wmi: Improve DSTS WMI method ID detection

The DSTS method detection mistakenly selects DCTS instead of DSTS if
nothing is returned when the method ID is not defined in WMNB. As a result,
the control of keyboard backlight is not functional for TUF Gaming series
laptops. Implement detection based on _UID of the WMI device instead.

There is evidence that DCTS is handled by ACPI WMI devices that have _UID
ASUSWMI, whereas none of the devices without ASUSWMI respond to DCTS and
DSTS is used instead [1].

DSDT examples:

FX505GM (_UID ATK):
Method (WMNB, 3, Serialized)
{ ...
    If ((Local0 == 0x53545344))
    {
        ...
        Return (Zero)
    }
    ...
    // No return
}

K54C (_UID ATK):
Method (WMNB, 3, Serialized)
{ ...
    If ((Local0 == 0x53545344))
    {
        ...
        Return (0x02)
    }
    ...
    Return (0xFFFFFFFE)
}

[1] Link: https://lkml.org/lkml/2019/4/11/322

Signed-off-by: Yurii Pavlovskyi <yurii.pavlovskyi@gmail.com>
Suggested-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>