mmc: sh_mobile_sdhi: use .enable_dma

tmio_mmc_dma has .enable_dma callback now.
let's use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>