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/chan_alloc.c b/openbsc/src/chan_alloc.c
index 4809496..d6e4ff1 100644
--- a/openbsc/src/chan_alloc.c
+++ b/openbsc/src/chan_alloc.c
@@ -396,7 +396,7 @@
 	return NULL;
 }
 
-struct gsm_lchan *lchan_for_subscr(struct gsm_subscriber *subscr)
+struct gsm_subscriber_connection *connection_for_subscr(struct gsm_subscriber *subscr)
 {
 	struct gsm_bts *bts;
 	struct gsm_network *net = subscr->net;
@@ -405,7 +405,7 @@
 	llist_for_each_entry(bts, &net->bts_list, list) {
 		lchan = lchan_find(bts, subscr);
 		if (lchan)
-			return lchan;
+			return &lchan->conn;
 	}
 
 	return NULL;