mavlink: Do not miss a vehicle_command

If 2 or more vehicle_command are queued a call to update() will
return the oldest vehicle_command and set the _cmd_time to the
timestamp of the last vehicle_command queued losing it.
Using update_if_changed() fix this causing all item being consumed
one at each call of send().

src/modules/mavlink/mavlink_messages.cpp

@@ -461,7 +461,7 @@ protected:
 		struct vehicle_command_s cmd;
 		bool sent = false;
 
-		if (_cmd_sub->update(&_cmd_time, &cmd)) {
+		if (_cmd_sub->update_if_changed(&cmd)) {
 
 			if (!cmd.from_external) {
 				if (_mavlink->verbose()) {