ixgbe: fix X540 ethtool loopback test.

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>