ACPI thermal: extract return values using PTR_ERR

Need to extract errors using PTR_ERR macro and
process accordingly.thermal_cooling_device_register
returning NULL means that CONFIG_THERMAL=n and in that
case no need to create symbolic links.

Signed-off-by: Thomas Sujith <sujith.thomas@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>