Now we can kill the embedding of net_device into hdlc_device.  Indeed,
all instances of hdlc_device are created by alloc_hdlcdev() and nothing
uses hdlc->netdev directly.  So we can
        * remove hdlc->netdev
        * have alloc_hdlcdev() implemented via alloc_netdev() with the rest
of hdlc_device as private part of net_device.
        * replace free_hdlcdev() with free_netdev().
        * have dev_to_hdlc(dev) simply return netdev_priv(dev).