Use osmo_tdef to implement dl-tbf-idle-time

Change-Id: I5e4f0d2f90e643600b7752525d6c2830856c9d3b
diff --git a/src/bts.cpp b/src/bts.cpp
index 2423400..a8760ae 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -84,6 +84,7 @@
 	{ .T=-2001, .default_val=2,   .unit=OSMO_TDEF_S,  .desc="PACCH assignment timer (s)",               .val=0 },
 	{ .T=-2002, .default_val=200, .unit=OSMO_TDEF_MS, .desc="Waiting after IMM.ASS confirm timer (ms)", .val=0 },
 	{ .T=-2030, .default_val=60,  .unit=OSMO_TDEF_S,  .desc="Time to keep an idle MS object alive (s)", .val=0 }, /* slightly above T3314 (default 44s, 24.008, 11.2.2) */
+	{ .T=-2031, .default_val=2000, .unit=OSMO_TDEF_MS, .desc="Time to keep an idle DL TBF alive (ms)",  .val=0 },
 	{ .T=0, .default_val=0, .unit=OSMO_TDEF_S, .desc=NULL, .val=0 } /* empty item at the end */
 };