igb: implement igb_ptp_suspend

Make igb_ptp_stop take advantage of this new function to reduce code
duplication.

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>