s390/qeth: fix setting VIPA address

commit 5f78e29c ("qeth: optimize IP handling in rx_mode callback")
restructured the internal address handling.
This work broke setting a virtual IP address.
The command
echo 10.1.1.1 > /sys/bus/ccwgroup/devices/<device>/vipa/add4
fails with file exist error even if the IP address has not
been set before.

It turned out that the search result for the IP address
search is handled incorrectly in the VIPA case.

This patch fixes the setting of an virtual IP address.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>