keep 'silent call' state in struct lchan

This enables us to reliably detect if a lchan is part of a silent
call or not.
diff --git a/openbsc/src/chan_alloc.c b/openbsc/src/chan_alloc.c
index 8141e47..48f87f2 100644
--- a/openbsc/src/chan_alloc.c
+++ b/openbsc/src/chan_alloc.c
@@ -302,6 +302,8 @@
 	for (i = 0; i < ARRAY_SIZE(lchan->neigh_meas); i++)
 		lchan->neigh_meas[i].arfcn = 0;
 
+	lchan->silent_call = 0;
+
 	/* FIXME: ts_free() the timeslot, if we're the last logical
 	 * channel using it */
 }