PM / QoS: Use NULL pointer instead of plain integer in qos.c

Fix the following sparse warning:
drivers/base/power/qos.c:465:29: warning: Using plain integer as NULL pointer
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>

drivers/base/power/qos.c

@@ -462,7 +462,7 @@ EXPORT_SYMBOL_GPL(dev_pm_qos_add_ancestor_request);
 static void __dev_pm_qos_drop_user_request(struct device *dev)
 {
 	dev_pm_qos_remove_request(dev->power.pq_req);
-	dev->power.pq_req = 0;
+	dev->power.pq_req = NULL;
 }
 
 /**