i40evf: program RSS LUT correctly

A recent change broke the RSS LUT programming, causing it to be
programmed with all 0. Correct this by actually assigning the
incremented value back to the counter variable so that the increment
will be remembered by the calling function.

While we're at it, add a proper kernel-doc function comment to our
helper function.

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