i40e: print message for pre-production hardware

The driver and hardware are not expected to work correctly
with revision_id 0 hardware.  Don't prevent the user from
using it, but be sure to print a warning.

Change-ID: I3712d34752bfad458078a5f35dfd0aa0ae9fd20e
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>