mtd: tests: nandbiterrs: Fix read_page return value

The number of corrected bitflips is not correctly reported by
the test until the bitflip threshold is reached.

read_page() shall return the number of corrected bitflips, but
mtd_read() returns 0 or a negative error, so we can't forward
its return value. In the absence of an error we always have
calculate the number of bitflips ourselves.
Signed-off-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NBoris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/tests/nandbiterrs.c

@@ -151,7 +151,7 @@ static int read_page(int log)
 	memcpy(&oldstats, &mtd->ecc_stats, sizeof(oldstats));
 
 	err = mtd_read(mtd, offset, mtd->writesize, &read, rbuffer);
-	if (err == -EUCLEAN)
+	if (!err || err == -EUCLEAN)
 		err = mtd->ecc_stats.corrected - oldstats.corrected;
 
 	if (err < 0 || read != mtd->writesize) {