thermal: trip: Fold __thermal_zone_get_trip() into its caller

Because __thermal_zone_get_trip() is only called by thermal_zone_get_trip()
now, fold the former into the latter.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/22339769.EfDdHjke4D@rjwysocki.net