GCS_MAVLink: use RADIO_STATUS message txbuf field

this avoids overrunning radio buffer in queued_param_send()

libraries/GCS_MAVLink/GCS.h

@@ -334,6 +334,7 @@ public:
 
     virtual uint64_t capabilities() const;
     uint16_t get_stream_slowdown_ms() const { return stream_slowdown_ms; }
+    uint8_t get_last_txbuf() const { return last_txbuf; }
 
     MAV_RESULT set_message_interval(uint32_t msg_id, int32_t interval_us);
 
@@ -571,6 +572,8 @@ private:
 
     // number of extra ms to add to slow things down for the radio
     uint16_t         stream_slowdown_ms;
+    // last reported radio buffer percent available
+    uint8_t          last_txbuf = 100;
 
     // perf counters
     AP_HAL::Util::perf_counter_t _perf_packet;

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -650,6 +650,8 @@ void GCS_MAVLINK::handle_radio_status(const mavlink_message_t &msg, bool log_rad
         last_radio_status.remrssi_ms = now;
     }
 
+    last_txbuf = packet.txbuf;
+
     // use the state of the transmit buffer in the radio to
     // control the stream rate, giving us adaptive software
     // flow control

libraries/GCS_MAVLink/GCS_Param.cpp

@@ -71,7 +71,7 @@ GCS_MAVLINK::queued_param_send()
     }
     count -= async_replies_sent_count;
 
-    while (count && _queued_parameter != nullptr) {
+    while (count && _queued_parameter != nullptr && get_last_txbuf() > 50) {
         char param_name[AP_MAX_NAME_SIZE];
         _queued_parameter->copy_name_token(_queued_parameter_token, param_name, sizeof(param_name), true);