Ignore packets above max portnum (511)

Meshtastic defines the maximum PortNum as 511.

https://buf.build/meshtastic/protobufs/docs/main:meshtastic

Packets using a PortNum above this are not currently useful
for mapping - ignore them for now.

src/mqtt.js

@@ -721,6 +721,7 @@ client.on("message", async (topic, message) => {
                     || portnum === 66 // ignore RANGE_TEST_APP
                     || portnum === 72 // ignore ATAK_PLUGIN
                     || portnum === 257 // ignore ATAK_FORWARDER
+                    || portnum > 511 // ignore above MAX
                 ){
                     return;
                 }