ACPI: thinkpad-acpi: check version of hot key firmware

Check the HKEY firmware version (HKEY.MHKV handler), and refuse to load if
it is unknown.  Use this instead of the presence of HKEY.DHKV to detect hot
key mask capability.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>