Because the ->get_trip_temp() and ->get_trip_type() thermal zone
callbacks are only invoked from __thermal_zone_get_trip() which is
always called by the thermal core under the zone lock, it is sufficient
for int340x_thermal_update_trips() to acquire the zone lock for mutual
exclusion with those callbacks.

Accordingly, modify int340x_thermal_update_trips() to use the zone lock
instead of the internal trip_mutex and drop the latter which is not
necessary any more.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>