spi: sh-msiof: Make sure all DMA operations have completed

In case of a bi-directional transfer, receive DMA may complete in the
rcar-dmac driver before transmit DMA, due to scheduling latencies.
As the MSIOF driver waits for completion of the receive DMA only, it may
submit the next transmit DMA request before the previous one has
completed.

Make the driver more robust by waiting for the completion of both
receive and transmit DMA, when applicable.

Based on a patch in the BSP by Ryo Kataoka.

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