mtd: rawnand: Only use u8 instead of uint8_t in nand_chip structure

Mechanical change to avoid using old types.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200529111322.7184-3-miquel.raynal@bootlin.com