mavlink: limit mavlink channels based on memory

src/modules/mavlink/mavlink_bridge_header.h

@@ -55,7 +55,7 @@
 #define MAVLINK_GET_CHANNEL_BUFFER mavlink_get_channel_buffer
 #define MAVLINK_GET_CHANNEL_STATUS mavlink_get_channel_status
 
-#if defined(__PX4_POSIX)
+#if !defined(CONSTRAINED_MEMORY)
 #define MAVLINK_COMM_NUM_BUFFERS 6
 #define MAVLINK_COMM_4 static_cast<mavlink_channel_t>(4)
 #define MAVLINK_COMM_5 static_cast<mavlink_channel_t>(5)

src/modules/mavlink/mavlink_main.h

@@ -543,7 +543,7 @@ private:
 
 	bool			_task_running{true};
 	static bool		_boot_complete;
-	static constexpr int	MAVLINK_MAX_INSTANCES{6};
+	static constexpr int	MAVLINK_MAX_INSTANCES{MAVLINK_COMM_NUM_BUFFERS};
 	static constexpr int	MAVLINK_MIN_INTERVAL{1500};
 	static constexpr int	MAVLINK_MAX_INTERVAL{10000};
 	static constexpr float	MAVLINK_MIN_MULTIPLIER{0.0005f};