i40e: fix kerneldoc for xsk methods

One method, xsk_umem_setup, had an incorrect kernel doc
description, which has been corrected.

Also fixes small typos found in the comments.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>