TSC: Add new gsm_ts_tsc() function to resolve TSC of TS

We don't want every caller to check for ts->tsc == -1 and then
using ts->trx->bts->tsc instead.  Rather, introduce a new inline
function to retrieve the correct value.
diff --git a/openbsc/src/libbsc/abis_nm.c b/openbsc/src/libbsc/abis_nm.c
index 717f56d..f491a0c 100644
--- a/openbsc/src/libbsc/abis_nm.c
+++ b/openbsc/src/libbsc/abis_nm.c
@@ -1678,10 +1678,7 @@
 			}
 		}
 	}
-	if (ts->tsc == -1)
-		msgb_tv_put(msg, NM_ATT_TSC, bts->tsc);	/* training sequence */
-	else
-		msgb_tv_put(msg, NM_ATT_TSC, ts->tsc);	/* training sequence */
+	msgb_tv_put(msg, NM_ATT_TSC, gsm_ts_tsc(ts));	/* training sequence */
 	if (bts->type == GSM_BTS_TYPE_BS11)
 		msgb_tlv_put(msg, 0x59, 1, &zero);