audioconvert: improve passthrough mode

When in passthrough mode, use the position io to update the
io_rate_match fields for the follower. This makes it possible for the
follower to also provide the right amount of data when the converter
is not selected in passthrough.

Add an option to configure the converter in None port config where it
removes all the ports. We can use this when removing the converter to
make sure all it's ports are removed.

When we remove the converter, make sure we expose the follower ports
directly so we can use them for passthrough.