libmsc/gsm_04_80.c: use gsm0480_create_release_complete()

The previous implementation of msc_send_ussd_release_complete() was
based on gsm0480_create_ussd_release_complete(), that doesn't
allow to specify GSM 04.07 transaction identifier.

The ability to specify particular transaction identifier
is required for handling multiple SS/USSD transactions.

Change-Id: Id2975c3383f18e83124ba38927c03980d67ddadb
Depends Change-Id: (libosmocore) Ie3ac85fcef90a5e532334ba3482804d5305c88d7
diff --git a/src/libmsc/msc_vty.c b/src/libmsc/msc_vty.c
index e1019a2..078b83a 100644
--- a/src/libmsc/msc_vty.c
+++ b/src/libmsc/msc_vty.c
@@ -1149,7 +1149,9 @@
 	}
 
 	msc_send_ussd_notify(conn, level, text);
-	msc_send_ussd_release_complete(conn);
+	/* FIXME: since we don't allocate a transaction here,
+	 * we use dummy GSM 04.07 transaction ID. */
+	msc_send_ussd_release_complete(conn, 0x00);
 
 	vlr_subscr_put(vsub);
 	talloc_free(text);