mtd: rawnand: oxnas: cleanup/simplify code

Simplify oxnas_nand_probe.

Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200724083825.GA31437@amd