SoundWire: multi-link support calls wait_for_completion() multiple times

[plbossart]

The premise for multi-link support is that the first link will start the bank switch on all links, using hardware-based sync. This is currently implemented, but we still call wait_for_completion() while dealing with the links remaining in the loop. This is not needed and can be optimized with a simple boolean flag.

@vinodkoul FYI