thermal: step_wise: cdev only needs update on a new target state

The cooling device only needs update on a new target state.  Since we
already check old target in thermal_zone_trip_update(), we can do one
more check to see if it's a new target state.  If not, we can reasonably
save some uncecesary code execution.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Eduardo Valentin <eduardo.valentin@ti.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/step_wise.c

@@ -133,6 +133,9 @@ static void thermal_zone_trip_update(struct thermal_zone_device *tz, int trip)
 		old_target = instance->target;
 		instance->target = get_target_state(instance, trend, throttle);
 
+		if (old_target == instance->target)
+			continue;
+
 		/* Activate a passive thermal instance */
 		if (old_target == THERMAL_NO_TARGET &&
 			instance->target != THERMAL_NO_TARGET)