sink-input, source-output: Assign to reference_ratio from a single place

sink-input, source-output: Assign to reference_ratio from a single place

This makes it easy to log a message every time the reference ratio
changes. I also need to add a hook for reference ratio changes, but
that need will go away if the stream relative volume controls will be
created by the core in the future.

Change-Id: I2344ba7825f76cd72241599bd138b21e16555e01