media: v4l: Memset argument to 0 before calling get_mbus_config pad op

Memset the config argument to get_mbus_config V4L2 sub-device pad
operation to zero before calling the operation. This ensures the callers
don't need to bother with it nor the implementations need to set all
fields that may not be relevant to them.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>