igb: Fix for DH89xxCC near end loopback test

On this chipset it is required to configure the MPHY block for loopback tests. If MPHY is not configured then all loopback tests will report failures.

Signed-off-by: Robert Healy <robert.healy@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>