mtd: rawnand: oxnas: Unregister all devices on error

On error, the oxnas probe path just frees the device which failed and
aborts the probe, leaving unreleased resources.

Fix this situation by calling mtd_device_unregister()/nand_cleanup()
on these.

Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-38-miquel.raynal@bootlin.com