igb: make serdes power down available for 82575 in addition to 82576 parts

There was a serdes power down workaround that was originally added for
82576 fiber.  However it has also been found that this workaround is needed
for serdes connections as well.  In addition it is also needed for 82575
serdes so we we need to remove the checks restricting it to 82576.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>