transaction: clarify magic 0xff transaction ID

Change-Id: I2d3a6334f49989bedbb1430d26ffad8b61dfd873
diff --git a/src/libmsc/transaction.c b/src/libmsc/transaction.c
index 237191d..23880aa 100644
--- a/src/libmsc/transaction.c
+++ b/src/libmsc/transaction.c
@@ -196,7 +196,7 @@
 	llist_for_each_entry(trans, &net->trans_list, entry) {
 		if (trans->vsub != vsub ||
 		    trans->protocol != protocol ||
-		    trans->transaction_id == 0xff)
+		    trans->transaction_id == TRANS_ID_UNASSIGNED)
 			continue;
 		used_tid_bitmask |= (1 << trans->transaction_id);
 	}