igb: rename igb_ptp_enable to igb_ptp_feature_enable

The name igb_ptp_enable is not synonymous with the purpose of this
function, so rename it to better explain its purpose.

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>