igb: Clear Go Link Disconnect for 82580 and later devices

Customers are requesting that the hw prevents PHY from establishing link
until the driver loads.  This patch clears the Go Link Disconnect bit which
provides the requested behavior on parts 82580 and later.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>