GCS_MAVLink: reject SET_MESSAGE_INTERVAL commands with p3 set

this is going to be used for changing the rate of a specific instance of a message at some stage

we have to reject it for now so that when the index is used the GCS is told that their message is invalid in this older version of the autopilot

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -3029,6 +3029,9 @@ MAV_RESULT GCS_MAVLINK::handle_command_do_aux_function(const mavlink_command_int
 
 MAV_RESULT GCS_MAVLINK::handle_command_set_message_interval(const mavlink_command_int_t &packet)
 {
+    if (!is_equal(packet.param3)) {
+        return MAV_RESULT_DENIED;
+    }
     return set_message_interval((uint32_t)packet.param1, (int32_t)packet.param2);
 }