mtd: nandsim: use nand_get_controller_data()

Commit d699ed25 ("mtd: nand: make use of
nand_set/get_controller_data() helpers") overlooked some uses of
nand_chip::priv.

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