i40e: Strip VEB stats if they are disabled in HW

Due to performance reasons, VEB stats have been disabled in the hw. This
patch adds code to check for that condition before accumulating these
stats.

Change-ID: I7d805669476fedabb073790403703798ae5d878e
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>