hysdn: Kill directly reference of netdev->priv

hysdn: Kill directly reference of netdev->priv

Usually, netdev->priv should point to the memory of private
data which is allocated in alloc_netdev().
netdev_priv() is used to get the address of the private data.

Change the netdev->priv pointer to another memory is wrong.
Use netdev->ml_priv for this case.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>