Drop PKI acks if there is no downlink on MQTTClientProxy (#8580)

* Discard everything if downlink isn't on

* Drop PKI packets when downlink not on

Author: RCGV1

src/mqtt/MQTT.cpp

@@ -66,6 +66,20 @@ inline void onReceiveProto(char *topic, byte *payload, size_t length)
           (strcmp(e.channel_id, channels.getGlobalId(ch.index)) == 0 && ch.settings.downlink_enabled))) {
         return;
     }
+
+    bool anyChannelHasDownlink = false;
+    size_t numChan = channels.getNumChannels();
+    for (size_t i = 0; i < numChan; ++i) {
+        const auto &c = channels.getByIndex(i);
+        if (c.settings.downlink_enabled) {
+            anyChannelHasDownlink = true;
+            break;
+        }
+    }
+
+    if (strcmp(e.channel_id, "PKI") == 0 && !anyChannelHasDownlink) {
+        return;
+    }
     // Generate node ID from nodenum for comparison
     std::string nodeId = nodeDB->getNodeId();
     if (strcmp(e.gateway_id, nodeId.c_str()) == 0) {