igb: Update get cable length function for i210/i211

There was a problem in the initial implementation of the get cable length
function for i210 and it did not work properly.  This patch fixes that
problem for i210/i211 devices.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>