mtd: nand: make use of mtd_to_nand() in NAND core code

mtd_to_nand() was recently introduced to avoid direct access to the
mtd->priv field. Update core code to use mtd_to_nand().

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>