mtd: core: fix nvmem error reporting

The master MTD will only have an associated device if
CONFIG_MTD_PARTITIONED_MASTER is set, thus we cannot use dev_err() on
mtd->dev. Instead use the parent device which is the physical flash
memory.

Fixes: 4b361cfa ("mtd: core: add OTP nvmem provider support")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230308082021.870459-2-michael@walle.cc