propagate RSL error cause codes to RR Channel Release cause

In various places that receive an error cause from RSL and place it in
lchan.release.rsl_error_cause, translate it to an RR cause and place that in
the recently added lchan.release.rr_cause. Hence the RR Channel Release message
now reflects more specific error causes when the reason for the error was
received in an RSL message's cause value.

Change-Id: I46eb12c91a8c08162b43dd22c7ba825ef3bbc6ac
diff --git a/include/osmocom/bsc/gsm_data.h b/include/osmocom/bsc/gsm_data.h
index 43d7040..c357981 100644
--- a/include/osmocom/bsc/gsm_data.h
+++ b/include/osmocom/bsc/gsm_data.h
@@ -1898,5 +1898,6 @@
 bool trx_has_valid_pchan_config(const struct gsm_bts_trx *trx);
 
 enum gsm48_rr_cause bsc_gsm48_rr_cause_from_gsm0808_cause(enum gsm0808_cause c);
+enum gsm48_rr_cause bsc_gsm48_rr_cause_from_rsl_cause(uint8_t c);
 
 #endif /* _GSM_DATA_H */