igb: Unset sigdetect for SERDES loopback on 82580 and i350

This patch unsets the sigdetect bit for SERDES loopback tests on 82580 and
i350 parts.  The loopback test can fail on these parts without this
setting.

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>