qeth: fix build error caused by VLAN changes

Adrian Bunk reports this build error:

CC      drivers/s390/net/qeth_l3_main.o
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/s390/net/qeth_l3_main.c:
In function 'qeth_l3_hard_start_xmit':
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/s390/net/qeth_l3_main.c:
2654: error: implicit declaration of function 'VLAN_TX_SKB_CB'
/home/bunk/linux/kernel-2.6/git/linux-2.6/drivers/s390/net/qeth_l3_main.c:
2654: error: invalid type argument of '->' (have 'int')
make[3]: *** [drivers/s390/net/qeth_l3_main.o] Error 1

The intention of the driver appears to be to invalidate the VLAN tag.
Change it to set skb->vlan_tci to zero, which has the same effect.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>