nat: Start using a write_queue for the BSC connection

We are still writing to the BSC directly and don't make real
use of this feature right now but we will need to do it.
diff --git a/openbsc/src/nat/bsc_nat_vty.c b/openbsc/src/nat/bsc_nat_vty.c
index 5430670..6959037 100644
--- a/openbsc/src/nat/bsc_nat_vty.c
+++ b/openbsc/src/nat/bsc_nat_vty.c
@@ -89,7 +89,7 @@
 	struct bsc_connection *con;
 	llist_for_each_entry(con, &_nat->bsc_connections, list_entry) {
 		vty_out(vty, "BSC lac: %d auth: %d fd: %d%s",
-			con->lac, con->authenticated, con->bsc_fd.fd, VTY_NEWLINE);
+			con->lac, con->authenticated, con->write_queue.bfd.fd, VTY_NEWLINE);
 	}
 
 	return CMD_SUCCESS;