mtd: nand: fix error handling in nand_prog_page_op() #1

On success chip->legacy.waitfunc() returns the NAND status byte, but on
failure it returns a negative error code. This was never tested for and
instead the return value was interpreted as NAND status without error
checking. Add the missing error check.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210205142725.13225-1-s.hauer@pengutronix.de