forcedeth: optimized routines

This patch breaks up the routines into two versions, one for legacy
descriptor versions (ver 1 and ver 2) and one for desc ver 3. This will
make the new desc functions more leaner and further reductions will be
made in next few patches.
Signed-Off-By: NAyaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>