spi: rspi: Configure DMA slave bus width to 8 bit

The new Renesas R-Car Gen2 DMA Controller driver (rcar-dmac) requires
explicit configuration of the DMA slave bus width.
Hardcode the DMA transfer size to 1 byte for both directions, as that's
the only supported configuration (16-bit DMA support was removed in
commit 9c5de2c1 ("spi: rspi: Remove
unused 16-bit DMA support")).

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