forcedeth: tx limiting

This patch optimizes the logic for tx limiting. It adds a flag to check
on the completion side instead of recalculating the number of empty
slots. Also, it removes the fields that were previous used for limiting
since they have no value.

Signed-Off-By: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>