mtd: nandsim: remove debugfs entries in error path

The debugfs entries must be removed before an error is returned in the
probe function. Otherwise another try to load the module fails and when
the debugfs files are accessed without the module loaded, the kernel
still tries to call a function in that module.

Fixes: 5346c27c ("mtd: nandsim: Introduce debugfs infrastructure")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>