e1000: only enable TSO6 via ethtool when using correct hardware

When enabling TSO via ethool on e1000, it is possible to set
NETIF_F_TSO6 on hardware that does not support it.  Setting TSO via
ethtool now matches the settings used when the hardware is probed.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>