mtd: rawnand: diskonchip: Fix the probe error path

Not sure nand_cleanup() is the right function to call here but in any
case it is not nand_release(). Indeed, even a comment says that
calling nand_release() is a bit of a hack as there is no MTD device to
unregister. So switch to nand_cleanup() for now and drop this
comment.

There is no Fixes tag applying here as the use of nand_release()
in this driver predates by far the introduction of nand_cleanup() in
commit d44154f9 ("mtd: nand: Provide nand_cleanup() function to free NAND related resources")
which makes this change possible. However, pointing this commit as the
culprit for backporting purposes makes sense even if it did not intruce
any bug.

Fixes: d44154f9

 ("mtd: nand: Provide nand_cleanup() function to free NAND related resources")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-mtd/20200519130035.1883-13-miquel.raynal@bootlin.com