mtd: phram: Fix error return code in phram_setup()

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: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210408133812.1209798-1-yukuai3@huawei.com