MavlinkOrbSubscription: optimize fields for size

reorder & reduce instance to uint8_t (which is more than enough).
Reduces sizeof(MavlinkOrbSubscription) from 40 to 24.

On Pixracer this frees almost 2KB of RAM

src/modules/mavlink/mavlink_orb_subscription.cpp

@@ -50,11 +50,11 @@
 MavlinkOrbSubscription::MavlinkOrbSubscription(const orb_id_t topic, int instance) :
 	next(nullptr),
 	_topic(topic),
-	_instance(instance),
 	_fd(-1),
+	_instance(instance),
 	_published(false),
-	_last_pub_check(0),
+	_subscribe_from_beginning(false),
-	_subscribe_from_beginning(false)
+	_last_pub_check(0)
 {
 }
 

src/modules/mavlink/mavlink_orb_subscription.h

@@ -95,11 +95,11 @@ public:
 
 private:
 	const orb_id_t _topic;		///< topic metadata
-	const int _instance;		///< get topic instance
 	int _fd;			///< subscription handle
+	const uint8_t _instance;		///< get topic instance
 	bool _published;		///< topic was ever published
-	hrt_abstime _last_pub_check;	///< when we checked last
 	bool _subscribe_from_beginning; ///< we need to subscribe from the beginning, e.g. for vehicle_command_acks
+	hrt_abstime _last_pub_check;	///< when we checked last
 
 	/* do not allow copying this class */
 	MavlinkOrbSubscription(const MavlinkOrbSubscription &);