mtd: rawnand: nandsim: Use an additional label when freeing the nandsim object

Cosmetic change to give a meaning to all labels in this complicated
error path.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-13-miquel.raynal@bootlin.com