mtd: rawnand: Use nand_to_mtd() in nand_{set,get}_flash_node()

Use the nand_to_mtd() helper to access chip->mtd as done everywhere
else.

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>