mtd: rawnand: Translate obscure bitfields into readable macros

Use the BIT() macro instead of defining a 8-digit value.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200507105241.14299-2-miquel.raynal@bootlin.com