lchan: Change transaction to work on the GSM Subscriber Connection

Change the MSC transaction code to operate on a GSM Subscriber Connection
instead of the lchan. This will help us to separate the two commands properly.
diff --git a/openbsc/src/mncc.c b/openbsc/src/mncc.c
index 01d59aa..afd5364 100644
--- a/openbsc/src/mncc.c
+++ b/openbsc/src/mncc.c
@@ -332,16 +332,16 @@
 	remote_trans = trans_find_by_callref(call->net, call->remote_ref);
 
 	/* this shouldn't really happen */
-	if (!remote_trans || !remote_trans->lchan) {
+	if (!remote_trans || !remote_trans->conn) {
 		LOGP(DMNCC, LOGL_ERROR, "No transaction or transaction without lchan?!?\n");
 		return -EIO;
 	}
 
 	/* RTP socket of remote end has meanwhile died */
-	if (!remote_trans->lchan->abis_ip.rtp_socket)
+	if (!remote_trans->conn->lchan->abis_ip.rtp_socket)
 		return -EIO;
 
-	return rtp_send_frame(remote_trans->lchan->abis_ip.rtp_socket, dfr);
+	return rtp_send_frame(remote_trans->conn->lchan->abis_ip.rtp_socket, dfr);
 }