media: video-mux: Parse information from firmware without using callbacks

Instead of using the convenience function
v4l2_async_register_fwnode_subdev(), parse the video-mux input endpoints
and set up the async sub-devices without using callbacks. The video-mux
knows which ports it must parse (the input ports) and how to handle
unconnected remotes, so it makes the code simpler to transfer control
of endpoint parsing to the driver.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>