audio_merge: Make audio channel mismatch message verbose

[kierank]

It's not an error, there are use-cases for this

[JDarnley]

Since I am working on the pipe I took a quick look at this. AFAICS if you do it in set_flow_def for the subpipes it will print one message for each input until the sum equals the output flow_def, then nothing when equal, then again when greater.

Maybe it is supposed to check that the input urefs match what the input flow_defs say and print a message when they are different indicating some error.

I'm not sure myself so I have just silenced it while I am testing.