ACPI: thermal: Use thermal_zone_device()

In order to get the device associated with the thermal zone, let's use
the wrapper thermal_zone_device() instead of accessing directly the
content of the thermal zone device structure.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
[ rjw: Subject adjustment, removal of trailing white space ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>