mtd: rawnand: omap2: fix device leak on probe failure

Make sure to drop the reference to the elm device taken by
of_find_device_by_node() during probe on errors and on driver unload.

Fixes: 62116e51 ("mtd: nand: omap2: Support for hardware BCH error correction.")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>