i40e: fix incorrect register definition

This register was defined incorrectly. Fix the increment value to 8, and
replace the iterator with _i to make the definition consistent with
other statistics registers.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>