Mavlink Receiver: Don't ack DO_SET_MODE commands (#27343)

01abe35563 introduced a handler for the
`MAV_CMD_DO_SET_MODE` enum, which directly forwards a vehicle command
into uORB. It did not disable immediate acknowledgement of this command directly in `mavlink_receiver`. This means that the command is always ack'd once with ACCEPTED prior to the ack issued from Commander

The fix is to set `send_ack=false` immediately after dispatching the
command message to uORB in mavlink receiver

src/modules/mavlink/mavlink_receiver.cpp

@@ -611,6 +611,7 @@ void MavlinkReceiver::handle_message_command_both(mavlink_message_t *msg, const
 
 	} else if (cmd_mavlink.command == MAV_CMD_DO_SET_MODE) {
 		_cmd_pub.publish(vehicle_command);
+		send_ack = false;	//Acknowledgement handled by Commander
 
 	} else if (cmd_mavlink.command == MAV_CMD_DO_AUTOTUNE_ENABLE) {