mtd: rawnand: au1550nd: Patch the read/write buf helper prototypes

To match the types passed by au1550nd_exec_instr() function.

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/20200419193037.1544035-5-boris.brezillon@collabora.com