mtd: rawnand: Avoid indirect access to ->data_buf()

The logic in nand_do_read_ops() is to use a bufpoi variable, either
set to the original buffer, or set to a bounce buffer which in the end
happens to be chip->data_buf depending on the value of the
use_bounce_buf boolean. This is not a reason to call chip->data_buf
directly when we know that we are using the bounce buffer. Let's use
bufpoi instead to be consistent.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200507105241.14299-7-miquel.raynal@bootlin.com