ACPI: thinkpad-acpi: prepare for NVRAM polling support

Make some small internal thinkpad-acpi changes to the hotkey subdriver code
that will make it easier to add NVRAM polling support.

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