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

firmware/usbstack/usb_audio.h

@@ -20,6 +20,7 @@
 #define USB_AUDIO_H
 
 #include "usb_ch9.h"
+#include "usb_class_driver.h"
 
 /* NOTE
  *
@@ -276,4 +277,6 @@ int usb_audio_get_frames_dropped(void);
  */
 int usb_audio_get_cur_volume(void);
 
+bool usb_audio_get_active(void);
+
 #endif