[mgcp] Only patch RTP packets when they arrived on the RTP port

Do not attempt to patch RTCP packets...
diff --git a/openbsc/src/mgcp/mgcp_network.c b/openbsc/src/mgcp/mgcp_network.c
index 1f51233..ceeadb1 100644
--- a/openbsc/src/mgcp/mgcp_network.c
+++ b/openbsc/src/mgcp/mgcp_network.c
@@ -193,12 +193,14 @@
 		dest = !dest;
 
 	if (dest == DEST_NETWORK) {
-		patch_payload(endp->net_payload_type, buf, rc);
+		if (proto == PROTO_RTP)
+			patch_payload(endp->net_payload_type, buf, rc);
 		return udp_send(fd->fd, &endp->remote,
 			     proto == PROTO_RTP ? endp->net_rtp : endp->net_rtcp,
 			     buf, rc);
 	} else {
-		patch_payload(endp->bts_payload_type, buf, rc);
+		if (proto == PROTO_RTP)
+			patch_payload(endp->bts_payload_type, buf, rc);
 		return udp_send(fd->fd, &endp->bts,
 			     proto == PROTO_RTP ? endp->bts_rtp : endp->bts_rtcp,
 			     buf, rc);