thermal: ACPI: Constify acpi_thermal_zone_ops

Because thermal zone operations are now stored directly in struct
thermal_zone_device, acpi_thermal_zone_ops need not be modified by
the thermal core and so it can be const.

Adjust the code accordingly.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>