mavlink parameters: try to send 5 params at once on USB & UDP connections

This further speeds up param loading.

On Pixracer via USB, it's possible to send up to 11 at once before the TX
buffer gets full, so there is still enough free bandwidth left.

src/modules/mavlink/mavlink_parameters.h

@@ -91,6 +91,10 @@ protected:
 
 	void send(const hrt_abstime t);
 
+	/// send a single param if a PARAM_REQUEST_LIST is in progress
+	/// @return true if a parameter was sent
+	bool send_one();
+
 	int send_param(param_t param, int component_id = -1);
 
 	// Item of a single-linked list to store requested uavcan parameters