forcedeth: ethtool tx csum fix

This patch fixes the ethtool tx csum "set" command. A recent patch was
submitted to remove HW_CSUM and use IP_CSUM instead. Therefore, the
corresponding ethtool command should also be modified.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>