s390/qeth: add support for IPv6 TSO

This adds TSO6 support for L3 qeth devices.
Just like for standard IPv6 traffic, TSO6 doesn't use IP offload and
thus runs over the normal qeth_xmit() path.

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