infiniband: Kill directly reference of netdev->priv

This use of netdev->priv is wrong.
The right way is:
alloc_netdev() with no memory for private data.
make netdev->ml_priv to point to c2_dev.

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