greybus: greybus_protocols: svc: Add missing comment

All request/responses either have a structure representing them or a
comment saying the request/response payload doesn't exist.

The comment was missing for route create response message, add it.
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>

drivers/staging/greybus/greybus_protocols.h

@@ -804,6 +804,7 @@ struct gb_svc_route_create_request {
 	__u8	intf2_id;
 	__u8	dev2_id;
 };
+/* route create response has no payload */
 
 
 /* RAW */