mtd: rawnand: Avoid a typedef

In new code, the use of typedef is discouraged. Turn this one in the
raw NAND core into a regular enumeration.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200526195633.11543-3-miquel.raynal@bootlin.com