[PATCH] [NET] mv643xx: add workaround for HW checksum generation bug

The hardware checksum generator on the mv64xxx occasionally generates
an incorrect checksum.  This patch works around the issue and enables
hardware checksum generation.
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>