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/bsc_api.c b/openbsc/src/libbsc/bsc_api.c
index 78fee88..7563378 100644
--- a/openbsc/src/libbsc/bsc_api.c
+++ b/openbsc/src/libbsc/bsc_api.c
@@ -277,7 +277,7 @@
 
 	sapi = link_id & 0x7;
 	msg->lchan = conn->lchan;
-	msg->trx = msg->lchan->ts->trx;
+	msg->dst = msg->lchan->ts->trx->rsl_link;
 
 	/* If we are on a TCH and need to submit a SMS (on SAPI=3) we need to use the SACH */
 	if (allow_sach && sapi != 0) {