s390/qeth: robustify qeth_get_ip_version()

Instead of assuming that skb->data points to the Ethernet header, use
the right helper and struct to access the Ethertype field.

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