thermal: Add a thermal zone id accessor

In order to get the thermal zone id but without directly accessing the
thermal zone device structure, add an accessor.

Use the accessor in the hwmon_scmi and acpi_thermal.

No functional change intented.

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