mtd: rawnand: mtk: Use nand_controller_init() instead of open-coding it

nand_controller_init() has been added to simplify nand_controller
struct initialization. Use this function instead of duplicating the
logic.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>