osmux: Simplify and constify param passing

Parameters passed to several functions are rearrange to make code
simplier to follow, as well as dependencies between the different
functions. As a result, some parameters can be marked as const while
doing the change.

Change-Id: Idebd4a66630c16548f557632a54d6b7e1b3906fd
diff --git a/src/libosmo-mgcp/mgcp_protocol.c b/src/libosmo-mgcp/mgcp_protocol.c
index b4a4792..d92725a 100644
--- a/src/libosmo-mgcp/mgcp_protocol.c
+++ b/src/libosmo-mgcp/mgcp_protocol.c
@@ -330,7 +330,7 @@
 		return;
 
 	if (mgcp_conn_rtp_is_osmux(conn))
-		osmux_send_dummy(endp, conn);
+		osmux_send_dummy(conn);
 	else
 		mgcp_send_dummy(endp, conn);
 }