rtp_bridge: Demote a chatty ERROR log message to DEBUG level.

Not having a second leg of an MGCP endpoint is a normal situtation
and can't be treated as an ERROR message, especially not as an ERROR
message logged on every RTP packet. This happens routinely at
the beginning of call setup and we get tens of ERROR messages in
the logs for every call.

Change-Id: If741a742208772bda4e59236345d7ae650368d5a

src/libosmo-mgcp/mgcp_network.c

@@ -1310,8 +1310,8 @@ int mgcp_dispatch_rtp_bridge_cb(int proto, struct sockaddr_in *addr, char *buf,
 
 	/* There is no destination conn, stop here */
 	if (!conn_dst) {
-		LOGPCONN(conn, DRTP, LOGL_ERROR,
-			 "unable to find destination conn\n");
+		LOGPCONN(conn, DRTP, LOGL_DEBUG,
+			 "no connection to forward an incoming RTP packet to\n");
 		return -1;
 	}