staging: iio: core: Avoid NULL pointer de-ref in case indio_dev->setup_ops are not in use

Drivers may not need setup_ops at all, so let the core supply
some empty ops.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>