mtdblock: tolerate corrected bit-flips

mtd_read() may return -EUCLEAN in case of corrected bit-flips.This
particular condition should not be treated like an error.

Signed-off-by: Bang Li <libang.linuxer@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230314165653.252673-1-libang.linuxer@gmail.com