i40e/i40evf: unhide and enable to one prefena field

The PREFENA field in the receive host memory cache (RX-HMC)
must be visible in order to be set to 1 at driver init for
best performance.

Change-ID: I16b0bcd84cf56f4b6c938201ff5e954bee5a1992
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>