On X540 we need to set the MACC.FLU bit to 1 in order to force the link
up before entering MAC loopback. This is only used in the ethtool loopback
test, which was failing. This patch corrects it.
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com> Tested-by: Evan Swanson <evan.swanson@intel.com> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>