i40e: only enable PTP interrupt cause if PTP is enabled

We should not blindly enable the PTP interrupt flags for all PFs. We
should only enable the PTP interrupt in PFs which have enabled
PTP.

Change-ID: I051a17cae4c199a2f3cf7852266e27eda6630525
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>