mtd: rawnand: au1550nd: Stop using IO_ADDR_{R, W} in au_{read, write}_buf[16]()

We are about to re-use those for the exec_op() implementation which
will not rely on au1550_hwcontrol(). Let's patch those helpers to
simply use the iomem address stored in the context.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200419193037.1544035-2-boris.brezillon@collabora.com