ACPI / thermal: Use mode to enable/disable kernel thermal processing

As per documentation, "mode" sysfs interface should be able to
enable/disable thermal processing in the kernel, so that user space
is able to take more control.

Currently, ACPI thermal driver is not following this setting, so
modify it to match the interface documentation.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>