mtd: rawnand: mtk: Convert the driver to exec_op()

Let's convert the driver to exec_op() to have one less driver relying
on the legacy interface.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200518170321.321697-1-boris.brezillon@collabora.com