bsc_api: Fix a use after free error in the Clear Request path

The implementation of bsc_hack would call subscr_con_free before
the BSC API has had the chance to call gsm0808_clear to try to
release other channels. Fix that by adding a return value.
diff --git a/openbsc/src/osmo_msc.c b/openbsc/src/osmo_msc.c
index 1fad510..0ed973b 100644
--- a/openbsc/src/osmo_msc.c
+++ b/openbsc/src/osmo_msc.c
@@ -36,9 +36,10 @@
 		gsm411_sapi_n_reject(conn);
 }
 
-static void msc_clear_request(struct gsm_subscriber_connection *conn, uint32_t cause)
+static int msc_clear_request(struct gsm_subscriber_connection *conn, uint32_t cause)
 {
 	gsm0408_clear_request(conn, cause);
+	return 1;
 }
 
 static int msc_compl_l3(struct gsm_subscriber_connection *conn, struct msgb *msg,