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/gsm_04_08_utils.c b/openbsc/src/gsm_04_08_utils.c
index 4871f60..ed3f905 100644
--- a/openbsc/src/gsm_04_08_utils.c
+++ b/openbsc/src/gsm_04_08_utils.c
@@ -57,7 +57,7 @@
 	 * work that the caller no longer has to do */
 	if (trans) {
 		gh->proto_discr = trans->protocol | (trans->transaction_id << 4);
-		msg->lchan = trans->lchan;
+		msg->lchan = trans->conn->lchan;
 	}
 
 	if (msg->lchan) {