mtd: ubi: fix NVMEM over UBI volumes on 32-bit systems

A compiler warning related to sizeof(int) != 8 when calling do_div()
is triggered when building on 32-bit platforms.
Address this by using integer types having a well-defined size.

Fixes: 3ce48580

 ("mtd: ubi: provide NVMEM layer over UBI volumes")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>