mtd: rawnand: nandsim: Free partition names on error in ns_init()

The ns_init() function shall free the partition names allocated by
ns_get_partition_name() on error.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-9-miquel.raynal@bootlin.com