forcedeth: tx data path optimization

This patch optimizes the tx data paths and cleans up the code (removes
vlan from descr1/2 since only valid for desc3, changes to make code
easier to read, etc).
Signed-Off-By: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>