mtd: sh_flctl: fix warnings due to improper casts

Cast pointers to uintptr_t instead of unsigned int. This fixes warnings
on platforms where pointers have a different size than int.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>