s390/qeth: simplify card look-up on IP notification

On an IP event, current code tries to determine if the netdev belongs
to a L3 card by walking all qeth cards in the system, and then all of
their VLAN devices too. Short-cut the whole thing by identifying a L3
device through its netdev_ops.

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