mtd: rawnand: docg4: fix the probe function error path

nand_release() should not be called on an MTD device that has not been
registered. While it should work thanks to the checks done in
mtd_device_unregister() it's a bad practice to cleanup/release
something that has not previously been initialized/allocated.

Rework the error path to follow this rule.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>