i40e: use explicit cast from u16 to u8

Current implementation generates compilation warnings.

Change-ID: Icceefb50fe62aefaf90a64afb7192e08355a4ec5
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>