Use osmo_tdef to implement dl-tbf-idle-time

Change-Id: I5e4f0d2f90e643600b7752525d6c2830856c9d3b
diff --git a/src/tbf_dl.cpp b/src/tbf_dl.cpp
index 37bd3ee..12c098a 100644
--- a/src/tbf_dl.cpp
+++ b/src/tbf_dl.cpp
@@ -1235,11 +1235,13 @@
 bool gprs_rlcmac_dl_tbf::keep_open(unsigned fn) const
 {
 	int keep_time_frames;
+	unsigned long dl_tbf_idle_msec;
 
-	if (bts_data()->dl_tbf_idle_msec == 0)
+	dl_tbf_idle_msec = osmo_tdef_get(bts_data()->T_defs_pcu, -2031, OSMO_TDEF_MS, -1);
+	if (dl_tbf_idle_msec == 0)
 		return false;
 
-	keep_time_frames = msecs_to_frames(bts_data()->dl_tbf_idle_msec);
+	keep_time_frames = msecs_to_frames(dl_tbf_idle_msec);
 	return frames_since_last_drain(fn) <= keep_time_frames;
 }