igb: Improve cable length function for I210, etc.

Previously, the PHY-specific code to get the cable length for the
I210 internal and related PHYs was reporting the cable length of a
single pair and reporting it as the min, max, and total cable length.
Update it so that all four pairs are checked so the true min, max,
and average cable lengths are reported.

Signed-off-by: Joe Schultz <jschultz@xes-inc.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>