v4l2-subdev: Return -EOPNOTSUPP for unsupported pad type in call_get_frame_desc()

v4l2-subdev: Return -EOPNOTSUPP for unsupported pad type in call_get_frame_desc()

The `get_frame_desc()` operation should always be called on a source pad,
which is indicated by the `MEDIA_PAD_FL_SOURCE` flag. This patch adds a
check in `call_get_frame_desc()` to ensure that the `MEDIA_PAD_FL_SOURCE`
flag is set for the pad before invoking `get_frame_desc()`. If the pad is
not a source pad, the function will return an `-EOPNOTSUPP` error,
signaling that the operation is not supported on non-source pads.

Suggested-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>