igb: Enable auto-crossover during forced operation on 82580 and above.

Newer devices supported by igb can support auto-crossover detection in
forced operation modes. Enable this in the driver, rather than clobbering
this functionality in forced operation.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>