qeth: repair crash in qeth_l3_vlan_rx_kill_vid()

Commit efc73f4b "net: Fix memory leak - vlan_info struct" adds deletion of
VLAN 0 for devices with feature NETIF_F_HW_VLAN_FILTER. For driver
qeth these are the layer 3 devices. Usually there exists no
separate vlan net_device for VLAN 0. Thus the qeth functions
qeth_l3_free_vlan_addresses4() and qeth_l3_free_vlan_addresses6()
require an extra checking if function __vlan_find_dev_deep()
returns with a net_device.

Cc: stable@vger.kernel.org
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>