i40e: Change variable type to avoid typecheck failure

Change the variable type to avoid compiler warning about int to u16
possible data truncation.

Change-ID: I5eb3b578c86513c9625ca32d2f0b57cc01d7dc98
Signed-off-by: Kevin Scott <kevin.c.scott@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>