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
5 files changed