i40e/i40evf: Add legacy-rx private flag to allow fallback to old Rx flow

This patch adds a control which will allow us to toggle into and out of the
legacy Rx mode.  The legacy Rx mode is what we currently do when performing
Rx.  As I make further changes what should happen is that the driver will
fall back to the behavior for Rx as of this patch should the "legacy-rx"
flag be set to on.

Change-ID: I0342998849bbb31351cce05f6e182c99174e7751
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>