forcedeth: sideband management fix

This patch contains a fix that implements proper communication with the
sideband management unit. Also, it makes sure that the speed is
correctly set for gigabit phys in the case where sideband mgmt unit
initialized the phy. Refer to bug #7684 for more details.

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