atl1: use csum_start

Use skb->csum_start for tx checksum offload preparation. Also swap
the variables css and cso so they hold the intended values of csum
start and offset, respectively.

Signed-off-by: Jay Cliburn <jacliburn@bellsouth.net>
Acked-by: Chris Snook <csnook@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>