mtd: rawnand: Leave chip->IO_ADDR_{R, W} to NULL when unused

There's no point in poisoning the ->IO_ADDR_{R,W}, a NULL pointer
is just as good to detect unexpected ->IO_ADDR_{R,W} usage.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>