forcedeth: phy status fix

The driver needs to ack only the phy status bits that it is currently
handling and preserve the other bits for the other handlers. For
example, when reading/writing from the phy, it should not clear the link
change interrupt bit. This will cause a missing link change interrupt.

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