igb: fixes 82576 serdes init to correctly support manual flow control changes

This patch changes the PCS configuration for serdes so that the flow
control options change be set via the ethtool -A option.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>