ACPI: thinkpad-acpi: prepare for device model conversion

Prepare the thinkpad-acpi driver for the conversion to the device
model, by renaming variables and doing other glue work that shall
make the later patches much cleaner.

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