According to MXM 2.1 specification, there is the only one instance of the
WMI GUID F6CB5C3C-9CAE-4EBD-B577-931EA32A2CC0 and so it is instance 0x0.

MXM 2.1 specification:
https://lekensteyn.nl/files/docs/mxm-2.1-software-spec.pdf

_WDG dump:
// Methods GUID {F6CB5C3C-9CAE-4EBD-B577-931EA32A2CC0}
0x3C, 0x5C, 0xCB, 0xF6, 0xAE, 0x9C, 0xBD, 0x4E, 0xB5, 0x77, 0x93,
0x1E, 0xA3, 0x2A, 0x2C, 0xC0,
0x4D, 0x58,    // Object ID "MX" = method "WMMX"
1,             // Instance Count
0x02,          // Flags (WMIACPI_REGFLAG_METHOD)
Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>