chan_alloc: Allocate a subscriber connection as child of the network

Due handover we might leave the BTS and if we ever allocate/release
a BTS dynamically we have a problem here.
diff --git a/openbsc/src/chan_alloc.c b/openbsc/src/chan_alloc.c
index a0eda54..3c8082d 100644
--- a/openbsc/src/chan_alloc.c
+++ b/openbsc/src/chan_alloc.c
@@ -513,7 +513,7 @@
 {
 	struct gsm_subscriber_connection *conn;
 
-	conn = talloc_zero(lchan->ts->trx->bts, struct gsm_subscriber_connection);
+	conn = talloc_zero(lchan->ts->trx->bts->network, struct gsm_subscriber_connection);
 	if (!conn)
 		return NULL;