i40iw: Simplify code to set fragments in SQ WQE

Replace a subtract and multiply with an add; while populating fragments
in SQ wqe.

Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Signed-off-by: Shiraz Saleem <shiraz.saleem@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>