igb: introduce IGB_PTP_OVERFLOW_CHECK flag

Don't continue to use complex MAC type checks for handling various cases
where we have overflow check code. Make this code more obvious by
introducing a flag which is enabled for hardware that needs these
checks.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>