mmc: tmio-mmc: Improve DMA stability on sh-mobile

On some SDHI tmio implementations the order of DMA and command completion
interrupts swaps, which leads to malfunction. This patch postpones
DMA activation until the MMC command completion IRQ time.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Chris Ball <cjb@laptop.org>