e1000e: Restrict MDIO Slow Mode workaround to relevant parts

It has been determined that the workaround of putting the PHY into MDIO
slow mode to access the PHY id is not necessary with Lynx Point and newer
parts.  The issue that necessitated the workaround has been fixed on the
newer hardware.

We will maintains, as a last ditch attempt, the conversion to MDIO Slow
Mode in the failure branch when attempting to access the PHY id so as to
cover all contingencies.

Signed-off-by: Dave Ertman <davidx.m.ertman@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>