APMFirmwarePlugin, RCRSSIIncidator: solve bug where no valid rssi would show indicator:

There was a bug scaling rc rssi from rc_channels mavlink
message where it was scaled to 0-100 from 0-255, and this
was wrong because 255 is meant to be invalid, and AP uses

0-254 anyway. We must leave 255 as it is so the frontend
can understand if we don't have data.

Also, in RCRSSIIndicator, we don't show the indicator anymore
if data is not valid, there is no point

src/FirmwarePlugin/APM/APMFirmwarePlugin.cc

@@ -954,9 +954,10 @@ void APMFirmwarePlugin::_handleRCChannels(Vehicle* vehicle, mavlink_message_t* m
         mavlink_rc_channels_t   channels;
 
         mavlink_msg_rc_channels_decode(message, &channels);
-        //-- Ardupilot uses 0-255 to indicate 0-100% where QGC expects 0-100
+        //-- Ardupilot uses 0-254 to indicate 0-100% where QGC expects 0-100
-        if(channels.rssi) {
+        // As per mavlink specs, 255 means invalid, we must leave it like that for indicators to hide if no rssi data
-            channels.rssi = static_cast<uint8_t>(static_cast<double>(channels.rssi) / 255.0 * 100.0);
+        if(channels.rssi && channels.rssi != 255) {
+            channels.rssi = static_cast<uint8_t>(static_cast<double>(channels.rssi) / 254.0 * 100.0);
         }
         MAVLinkProtocol* mavlink = qgcApp()->toolbox()->mavlinkProtocol();
         mavlink_msg_rc_channels_encode_chan(

src/ui/toolbar/RCRSSIIndicator.qml

@@ -24,7 +24,7 @@ Item {
     anchors.top:    parent.top
     anchors.bottom: parent.bottom
 
-    property bool showIndicator: _activeVehicle.supportsRadio
+    property bool showIndicator: _activeVehicle.supportsRadio && _rcRSSIAvailable
 
     property var    _activeVehicle:     QGroundControl.multiVehicleManager.activeVehicle
     property bool   _rcRSSIAvailable:   _activeVehicle ? _activeVehicle.rcRSSI > 0 && _activeVehicle.rcRSSI <= 100 : false