media: docs: uAPI: Expand error documentation for invalid 'which' value

Multiple subdev ioctls that take a 'which' field do not document the
error returned when the field has an invalid value. Expand the
documentation to fix this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>