s390_net: Kill directly reference of netdev->priv

The private data comes from ccwgroup_device.
So just don't allocate private data memory when do alloc_netdev()
and use netdev->ml_priv to reference private data.

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