e1000e: disable K1 on PCH LOM when in PHY loopback mode

When performing the ethtool PHY loopback test on PCH-based LOMs (82577 and
82578), disable K1 (a MAC-PHY interconnect low power mode) otherwise
packets might get corrupted.

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>