ACPI: thinkpad-acpi: clean up CMOS commands subdriver

Some ThinkPad CMOS commands subdriver cleanups, and also rename/promote
cmos_eval to a ACPI helper function, as it is used by many other
subdrivers.

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