media: pci: mg4b: fix uninitialized iio scan data

Fix potential leak of uninitialized stack data to userspace by ensuring
that the `scan` structure is zeroed before use.

Fixes: 0ab13674 ("media: pci: mgb4: Added Digiteq Automotive MGB4 driver")
Cc: stable@vger.kernel.org
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>