netdevice hdlc: Convert directly reference of netdev->priv

For killing directly reference of netdev->priv, use netdev->ml_priv to replace it.
Because the private pvc data comes from add_pvc() and can't be allocated in
alloc_netdev().

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>