netdevice: 82596: Convert netdev->priv to netdev_priv

1. When alloc_etherdev(), no memory be allocated to netdev->priv.
2. And it's need to get a whole page for priv.

For these reasons, use netdev->ml_priv to point to the page is the
best method to convert directly reference of netdev->priv.

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