usbaudio: block playback while usbaudio is active
Needed due to us commandeering the AUDIO DSP "channel" for USBAudio.
If/When we add another DSP channel, this can be reverted.
Works, but still goes to a "blank" WPS screen for a split second before
cancelling out.
Change-Id: I5fb8a1e226b4d3e46c86c59d593d807f49d7a35f