igb: M88E1543 PHY downshift implementation

This patch implements downshift mechanism for M88E1543 PHY, so that
downshift is disabled first during link setup process, and later enabled
if we are master and downshift link is negotiated. Also cleaned up
return code implementation.

Signed-off-by: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>