mtd: spinand: read returns badly if the last page has bitflips

In case of the last page containing bitflips (ret > 0),
spinand_mtd_read() will return that number of bitflips for the last
page while it should instead return max_bitflips like it does when the
last page read returns with 0.

Signed-off-by: Weixiong Liao <liaoweixiong@allwinnertech.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: stable@vger.kernel.org
Fixes: 7529df46

 ("mtd: nand: Add core infrastructure to support SPI NANDs")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>