mmc: bcm283x: fix int to pointer cast

On build with 32 bit, there is a warning for int-to-pointer-cast.
Fix the int to pointer cast by using uintptr_t.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>