tg3: 5785: Set port mode to MII when link down

This patch sets the port mode to MII when the link is down for the 5785.
Setting the port mode to MII instead of GMII saves power.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>