PM / QoS: Use NULL pointer instead of plain integer in pm_qos.h

Fix the following sparse warning:
include/linux/pm_qos.h:69:28: 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>

include/linux/pm_qos.h

@@ -66,7 +66,7 @@ enum pm_qos_req_action {
 
 static inline int dev_pm_qos_request_active(struct dev_pm_qos_request *req)
 {
-	return req->dev != 0;
+	return req->dev != NULL;
 }
 
 int pm_qos_update_target(struct pm_qos_constraints *c, struct plist_node *node,