bsc_api: Rename lchan_for_subscr to connection_for_subscr and update code

With handover and late/early assignment there might be two channels
for one subscriber and only the BSC knows which one to use, so use
the gsm_subscriber_connection everywhere...
diff --git a/openbsc/src/silent_call.c b/openbsc/src/silent_call.c
index 8bd5341..00acc1d 100644
--- a/openbsc/src/silent_call.c
+++ b/openbsc/src/silent_call.c
@@ -128,15 +128,13 @@
 /* end a silent call with a given subscriber */
 int gsm_silent_call_stop(struct gsm_subscriber *subscr)
 {
-	struct gsm_lchan *lchan;
 	struct gsm_subscriber_connection *conn;
 
-	lchan = lchan_for_subscr(subscr);
-	if (!lchan)
+	conn = connection_for_subscr(subscr);
+	if (!conn)
 		return -EINVAL;
 
 	/* did we actually establish a silent call for this guy? */
-	conn = &lchan->conn;
 	if (!conn->silent_call)
 		return -EINVAL;