igb: SERDES loopback sigdetect bit on i210 devices

This patch implements SERDES loopback configuration for i210 devices by
unsetting sigdetect bit, so as to fix Ethtool loopback test failure. Old
sigdetect code is also simplified to take care of all devices newer than 82580

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>