gsm_04_80: Send a Release Complete otherwise the USSD unit stays BUSY

We need to release the USSD unit, otherwise it is staying blocked
and will stop to function (even across LUs on my a1200). This code
should encode the transaction and the direction depending on the
network state but this is omitted right now.

openbsc/include/openbsc/gsm_04_80.h

@@ -30,5 +30,6 @@ int gsm0480_wrap_invoke(struct msgb *msg, int op, int link_id);
 int gsm0480_wrap_facility(struct msgb *msg);
 
 int gsm0480_send_ussdNotify(struct gsm_subscriber_connection *conn, const char *text);
+int gsm0480_send_releaseComplete(struct gsm_subscriber_connection *conn);
 
 #endif

openbsc/src/gsm_04_80.c

@@ -469,3 +469,19 @@ int gsm0480_send_ussdNotify(struct gsm_subscriber_connection *conn, const char *
 
 	return gsm0808_submit_dtap(conn, msg, 0);
 }
+
+int gsm0480_send_releaseComplete(struct gsm_subscriber_connection *conn)
+{
+	struct gsm48_hdr *gh;
+	struct msgb *msg;
+
+	msg = gsm48_msgb_alloc();
+	if (!msg)
+		return -1;
+
+	gh = (struct gsm48_hdr *) msgb_push(msg, sizeof(*gh));
+	gh->proto_discr = GSM48_PDISC_NC_SS;
+	gh->msg_type = GSM0480_MTYPE_RELEASE_COMPLETE;
+
+	return gsm0808_submit_dtap(conn, msg, 0);
+}

openbsc/src/vty_interface_layer3.c

@@ -408,6 +408,7 @@ DEFUN(subscriber_ussd_notify,
 	}
 
 	gsm0480_send_ussdNotify(conn, text);
+	gsm0480_send_releaseComplete(conn);
 
 	subscr_put(subscr);
 	talloc_free(text);