core: fix another double promise

The problem is that the function called when a timeout happens, always
calls the _all_params_callback which it should not. In fact, there
should be two different timeout handlers instead.

src/mavsdk/core/mavlink_parameters.cpp

@@ -913,6 +913,7 @@ void MAVLinkParameters::process_param_value(const mavlink_message_t& message)
             if (param_value.param_index + 1 == param_value.param_count) {
                 _parent.unregister_timeout_handler(_all_params_timeout_cookie);
                 _all_params_callback(_all_params);
+                _all_params_callback = nullptr;
             } else {
                 _parent.unregister_timeout_handler(_all_params_timeout_cookie);