bsc_subscr_conn_fsm: use proper cause values in SAPI N REJECT

As 3GPP TS 48.058 clearly states, an RLL RELease INDication is only
sent if the datalink is released by the MS.  Thre's no cause value
giving further diagnostics, but if the MS releases the link, it is
clearly not the BSS's fault, and hence "BSS not equipped" is wrong.

Change-Id: I38222e60071841abcd06046a472ddb35907164a4
Related: OS#4728
diff --git a/tests/bsc/bsc_test.c b/tests/bsc/bsc_test.c
index 86d8972..d999608 100644
--- a/tests/bsc/bsc_test.c
+++ b/tests/bsc/bsc_test.c
@@ -223,7 +223,7 @@
 	OSMO_ASSERT(0);
 }
 
-void bsc_sapi_n_reject(struct gsm_subscriber_connection *conn, uint8_t dlci) {}
+void bsc_sapi_n_reject(struct gsm_subscriber_connection *conn, uint8_t dlci, enum gsm0808_cause cause) {}
 void bsc_cipher_mode_compl(struct gsm_subscriber_connection *conn, struct msgb *msg, uint8_t chosen_encr) {}
 int bsc_compl_l3(struct gsm_subscriber_connection *conn, struct msgb *msg, uint16_t chosen_channel)
 { return 0; }