fix bug in rssi assignment

src/drivers/px4fmu/fmu.cpp

@@ -713,8 +713,12 @@ void PX4FMU::fill_rc_in(uint16_t raw_rc_count,
 	_rc_in.timestamp_last_signal = _rc_in.timestamp_publication;
 	_rc_in.rc_ppm_frame_length = 0;
 
-	/* don't touch rssi if no value was provided */
-	if (rssi >= 0) {
+	/* fake rssi if no value was provided */
+	if (rssi == -1) {
+		_rc_in.rssi =
+			(!frame_drop) ? RC_INPUT_RSSI_MAX : (RC_INPUT_RSSI_MAX / 2);
+
+	} else {
 		_rc_in.rssi = rssi;
 	}