mtd: rawnand: cs553x: Implement exec_op()

So we can later get rid of the legacy hooks.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200501090650.1138200-4-boris.brezillon@collabora.com