mmc: sh_mmcif: reset DMA completion immediately before starting DMA

DMA completion can be signalled from the DMA callback and from the error
handler. If both are called, the completion struct can enter an
inconsistent state. To prevent this move completion initialisation
immediately before activating DMA.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>