forcedeth: add hardware tx checksumming

Recent forcedeth nics support checksum offloading for tx.

The attached patch, written by Ayaz Abdulla, adds the support to the
driver.

It also cleans up the handling of the three dma ring entry formats that
are supported by the driver.

Signed-off-By: Manfred Spraul <manfred@colorfullife.com>
Signed-off-By: Jeff Garzik <jgarzik@pobox.com>