vpif_capture relied on a 1-1 mapping of input and subdev. This is not
necessarily the case. Separate the two. So there is a list of subdevs
and a list of inputs. Each input refers to a subdev and has routing
information. An input does not have to have a subdev.
The initial input for each channel is set to the fist input.
Currently missing is support for associating multiple subdevs with
an input.