usbaudio: send through dsp (new)

Does not seem to affect UI usability, but allowable DSP loads will vary
based on device and playback sample rate.

To-Do (someday):
- Add dedicated DSP channel
- How to apply DSP settings to above new channel? (UI)
- How to lock out timestretch from being enabled?

Change-Id: Ia24d1055340354e2c32e6008e7e2b03a8e88867d

apps/plugins/mpegplayer/audio_thread.c

@@ -665,7 +665,7 @@ static void audio_thread(void)
             }
 
             dst.bufcount = size / (2 * sizeof (int16_t));
-            rb->dsp_process(td.dsp, &td.src, &dst);
+            rb->dsp_process(td.dsp, &td.src, &dst, true);
 
             if (dst.remcount > 0)
             {