mgcp_network: refactor MGCP_DUMMY_LOAD

The constant MGCP_DUMMY_LOAD is used ambigously. Sometimes it is used as
initalizer for an array, sometimes it is used as a single byte. Also the
name is not very expressive. Lets refactor this.

Change-Id: I21d96cefeeb647958bfa1e22a0ea030884746fad
Related: OS#4005
diff --git a/src/libosmo-mgcp/mgcp_osmux.c b/src/libosmo-mgcp/mgcp_osmux.c
index bb85735..8f0a906 100644
--- a/src/libosmo-mgcp/mgcp_osmux.c
+++ b/src/libosmo-mgcp/mgcp_osmux.c
@@ -375,7 +375,7 @@
 	}
 
 	/* not any further processing dummy messages */
-	if (msg->data[0] == MGCP_DUMMY_LOAD)
+	if (mgcp_is_rtp_dummy_payload(msg))
 		return osmux_handle_dummy(cfg, &addr, msg);
 
 	rem = msg->len;