mtd: rawnand: nandsim: Fix the label pointing on nand_cleanup()

Drop the generic err_exit.

The remaining operation to do from this goto statement is to cleanup
the NAND allocations, so rename it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-15-miquel.raynal@bootlin.com