forcedeth: add clock gating feature <resend>

This patch adds new logic to support a clock gating feature found on the
latest set of chipsets. The clock gating is performed on the tx/rx
engines when the link is disconnected. Clock gating helps in reducing
power consumption.

* modified based on comments from netdev

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>