PM / QoS: Fix writing 'auto' to pm_qos_latency_tolerance_us

If it was already 'auto', then writing 'auto' again would
incorrectly fail.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/base/power/qos.c

@@ -856,7 +856,10 @@ int dev_pm_qos_update_user_latency_tolerance(struct device *dev, s32 val)
 		struct dev_pm_qos_request *req;
 
 		if (val < 0) {
-			ret = -EINVAL;
+			if (val == PM_QOS_LATENCY_TOLERANCE_NO_CONSTRAINT)
+				ret = 0;
+			else
+				ret = -EINVAL;
 			goto out;
 		}
 		req = kzalloc(sizeof(*req), GFP_KERNEL);