i40e/i40evf: Fix compile issue related to const string

Add const to functions that return strings that aren't going to be
modified. This addresses some reported compile complaints.

Change-ID: Ic56b1e814ab4d23a50480e7fdec652445f776ee8
Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>