mtd: rawnand: Fix comments about the use of bufpoi

Clarify these comments which are not very accurate (even wrong in the
read case).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200507105241.14299-5-miquel.raynal@bootlin.com