i40e: use priv flags to control flow director

Some customers wish to be able to control our hardware specific
feature called flow director, at runtime.  This patch enables
ethtool priv flags to control this driver/hardware specific feature.

ethtool --set-priv-flags ethX flow-director-atr off

NOTE: the ethtool ntuple interface controls the flow-director
      sideband rules.

Change-ID: Iba156350b07fa2ce66f53ded51739f9a3781fe0e
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>