forcedeth: tx collision fix

This patch supports a new fix in hardware regarding tx collisions. In
the cases where we are in autoneg mode and the link partner is in forced
mode, we need to setup the tx deferral register differently in order to
reduce collisions on the wire.

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