network/netdev: fix counter handling if request is cancelled

network/netdev: fix counter handling if request is cancelled

Follow-up for 1003093604661bd984574889167f2ff4dfd6209c.

If a netdev is detached for some reasons, then previously the request
was simply cancelled, and the underlying interface never enter the
configured state, as the 'stacked_netdevs_created' flag never set.

This makes the counter decremented manually by the function, and set the
flag. So, the underlying interface can eter the configured state.