qeth: use dev->ml_priv instead of dev->priv

From: Heiko Carstens <heiko.carstens@de.ibm.com>

This makes qeth working again after git commit
e3c50d5d
"netdev: netdev_priv() can now be sane again.".

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>