src: use new msg->dst pointer instead of deprecated msg->trx

This patch modifies openBSC code to use msg->dst which stores the
pointer to the signalling link structure instead of the pointer to
the transceiver structure.

This patch prepares the introduction of libosmo-abis.
diff --git a/openbsc/src/libbsc/gsm_04_08_utils.c b/openbsc/src/libbsc/gsm_04_08_utils.c
index 92335f2..968e62e 100644
--- a/openbsc/src/libbsc/gsm_04_08_utils.c
+++ b/openbsc/src/libbsc/gsm_04_08_utils.c
@@ -44,7 +44,7 @@
 static int gsm48_sendmsg(struct msgb *msg)
 {
 	if (msg->lchan)
-		msg->trx = msg->lchan->ts->trx;
+		msg->dst = msg->lchan->ts->trx->rsl_link;
 
 	msg->l3h = msg->data;
 	return rsl_data_request(msg, 0);