igb: Power down link when interface is down

This changes the behavior of the driver to power down the link
when the associated interface is down, unless management is enabled.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>