i40e: add functions to control default VSI

Add functions to enable and disable default VSI on a VEB. This allows
for configuration of limited promiscuous mode specifically for bridging
purposes.

Change-ID: I0cc5bd68b31c500fdff4d47e1f15d50d2739faf4
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>