mmc: sh_mmcif: Fix compilation warning on 64-bit platforms

Casting an integer to a void * generates a "cast to pointer from integer
of different size" warning. Cast the integer to an unsigned long first
to fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Chris Ball <cjb@laptop.org>