media: v4l2-subdev: Fix indentation in v4l2-subdev.h

Fix a simple indentation issue in the v4l2-subdev.h header.

Fixes: f57fa295

 ("media: v4l2-subdev: Add new ioctl for client capabilities")
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>