Remove ad-hoc handling of possible_in == 0

Remove ad-hoc handling of possible_in == 0

Just make the code responsible for possible_in > 0 also handle
possible_in == 0 since it nearly does the same thing.

The only difference is that the possible_in == 0 case didn't check at
all for audio_in, essentially acting as if possible_in was audio_in.
There is thus a small POLICY CHANGE, but the selected configuration will
stay the same unless a better matching configuration is available.