media: mc: fix endpoint iteration

When creating links from a subdev to a sink, the current logic tries to
iterate over the endpoints of dev's fwnode.

This might not be correct when the subdev uses a different fwnode
compared to the dev's fwnode.

If, when registering, the subdev's fwnode is not set, the code inside
v4l2_async_register_subdev will set it to the dev's fwnode.

To fix this, just use the subdev's fwnode.

Signed-off-by: Cosmin Tanislav <demonsingur@gmail.com>
Fixes: 0d3c81e8 ("media: v4l2-mc: add v4l2_create_fwnode_links helpers")
Cc: stable@vger.kernel.org
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>