mtd: rawnand: oxnas: Release all devices in the _remove() path

oxnans_nand_remove() should release all MTD devices and clean all NAND
devices, not only the first one registered.

Fixes: 66859249 ("mtd: nand: Add OX820 NAND Support")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-39-miquel.raynal@bootlin.com