mtd: nand: update mtd_to_nand()

Now that all drivers are using the mtd instance embedded in the nand_chip
struct we can safely update the mtd_to_nand() implementation to use
the container_of macro instead of returning the content of mtd->priv.
This will allow us to remove mtd->priv = chip assignments done in all
NAND controller drivers.

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