i40e: Tweak for-loop in i40e_ethtool.c

Tweak a for-loop to make it easier to add conditional stats in the future.

Signed-off-by: Catherine Sullivan <catherine.sullivan@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>