i40e: check I40E_FLAG_PTP before handling Tx or Rx timestamps

We should not be doing Tx or Rx timestamps if we do not have PTP
enabled. Add checks to ensure that we don't attempt to handle any PTP
related timestamping code if we have not enabled PTP on that PF.

Change-ID: I4335942ae2d5c5f91abfdbeeea02bcace49e7677
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>