mtd: rawnand: nandsim: Free the allocated device on error in ns_init()

The nandsim device is allocated and initialized inside ns_init() by a
call to ns_alloc_device(), free it on error.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-10-miquel.raynal@bootlin.com