igb: Explicitly label self-test result indices

Previously, the ethtool self-test gstrings/data arrays were accessed via
hardcoded indices, which made the code difficult to follow. This patch
replaces the hardcoded values with enum-based labels.

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>