nandsim: Fix return code testing of ns_find_operation()

ns_find_operation() returns 0 on success.

Fixes: 052a7a53 ("mtd: rawnand: nandsim: Clean error handling")
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200615113404.25447-1-richard@nod.at