s390/qeth: don't verify device when setting MAC address

There's no reason why l2_set_mac_address() should ever be called for
a netdevice that's not owned by qeth. It's certainly not required for
VLAN devices, which have their own netdev_ops.

Also:
1) we don't do such validation for any of the other netdev_ops routines.
2) the code in question clearly has never been actually exercised;
   it's broken. After determining that the device is not owned
   by qeth, it would still use dev->ml_priv to write a qeth trace entry.

Remove the check, and its helper that walked the global card list.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>