transaction: clarify magic 0xff transaction ID

Change-Id: I2d3a6334f49989bedbb1430d26ffad8b61dfd873
diff --git a/src/libmsc/gsm_09_11.c b/src/libmsc/gsm_09_11.c
index 08721d4..d2ad0b7 100644
--- a/src/libmsc/gsm_09_11.c
+++ b/src/libmsc/gsm_09_11.c
@@ -296,7 +296,7 @@
 
 	/* Allocate a new transaction */
 	trans = trans_alloc(net, vsub, GSM48_PDISC_NC_SS,
-		0xff, gsup_msg->session_id);
+		TRANS_ID_UNASSIGNED, gsup_msg->session_id);
 	if (!trans) {
 		LOGP(DMM, LOGL_ERROR, " -> No memory for trans\n");
 		return NULL;