mtd: core: Align comment with an action in mtd_otp_nvmem_add()

The comment is related to the non-error case, make it more clear
by inverting the condition. It also makes code neater at the end.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240325151150.3368658-1-andriy.shevchenko@linux.intel.com