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/utils/rs232.c b/openbsc/src/utils/rs232.c
index 4e35cb0..01b8c7e 100644
--- a/openbsc/src/utils/rs232.c
+++ b/openbsc/src/utils/rs232.c
@@ -128,7 +128,7 @@
 	if (!sh->rx_msg) {
 		sh->rx_msg = msgb_alloc(SERIAL_ALLOC_SIZE, "RS232 Rx");
 		sh->rx_msg->l2h = NULL;
-		sh->rx_msg->trx = sh->bts->c0;
+		sh->rx_msg->dst = sh->bts->c0->rsl_link;
 	}
 	msg = sh->rx_msg;