mtd: nand: sunxi: remove direct mtd->priv accesses

mtd->priv is no longer pointing to the struct nand_chip it is attached to.
Replace those accesses by mtd_to_nand() calls.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Fixes: 4be4e03e ("mtd: nand: sunxi: add randomizer support")
Signed-off-by: Brian Norris <computersforpeace@gmail.com>