With 76b6f74c53 we begin embedding a struct mtd_info into struct
nand_chip. With this mtd_to_nand() must be changed accordingly.

Fixes: 76b6f74c53 ("mtd: nand: Embed struct mtd_info into struct nand_chip")
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>