e1000e: MDIO slow mode should always be done for 82577

A previous 82577 workaround that set the MDIO access speed to slow mode for
every PHY register read/write when the cable is unplugged should instead
set the access mode to always be slow before any PHY register access.
Since the mode bit gets cleared when the PHY is reset, set the mode after
every PHY reset.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>