ACPI: thinkpad-acpi: cleanup thermal subdriver for sysfs conversion

Clean-up the thermal subdriver for sysfs conversion.  Make thermal_get_*
reentrancy-safe while at it, and add the missing thermal_read_mode variable
to the header file.

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