mmc: tmio: Configure DMA slave bus width

Even though some implementations support 4-bytes data register access,
the tmio driver never configures the hardware in such a way and always
performs 2-bytes accesses. Hardcode the DMA transfer size to 2 bytes.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>