[MTD] Fix build warnings in RedBoot MTD partition parser.

Fix build warnings from drivers/mtd/redboot.c due to
use of `unsigned long` in `struct fis_image_desc` for
fields being passed to swab32s() which expects __u32 *

Change the entries to uint32_t to make them compatible
with the swab32s() function

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>