thermal: trip: Constify thermal zone argument of thermal_zone_trip_id()

Because thermal_zone_trip_id() does not update the thermal zone object
passed to it, its pointer argument representing the thermal zone can be
const, so adjust its definition accordingly.

No functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>