spi: rspi: Pass sg_tables instead of spi_tranfer to rspi_*_dma()

The DMA routines only need access to the scatter-gather tables inside the
spi_transfer structures, hence just pass those.

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