spi: sh-msiof: Refactor sh_msiof_transfer_one()

  - Move buffer pointer and length setup to the top,
  - Make unsigned values unsigned,
  - Loop over words and increment pointers instead of recalculating them,
    which allows to kill bytes_done.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@linaro.org>