mmc: sh_mmcif: Remove .down_pwr() callback from platform data

The callback isn't used by the driver and isn't initialized by board
code. Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>