mtd: mtd_oobtest: fix error return code in mtd_oobtest_init()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210409010739.1021001-1-zhangxiaoxu5@huawei.com