ACPI: thinkpad-acpi: do not arm fan watchdog if it would not work

Do not enable/rearm the fan control safety watchdog if we would not be able
to do anything to the fan anyway.

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