Cleaned up some build warnings when NUM_USB_CHAN_OUT = 0

module_usb_audio/usb_buffer/decouple.xc

@@ -150,15 +150,15 @@ int g_maxPacketSize = MAX_DEVICE_AUD_PACKET_SIZE_IN_FS;
 #pragma unsafe arrays
 void handle_audio_request(chanend c_mix_out)
 {
-    int outSamps;
     int space_left;
 
     /* Input word that triggered interrupt and handshake back */
     unsigned underflowSample = inuint(c_mix_out);
 
 #if (NUM_USB_CHAN_OUT == 0)
-    outuint(c_mix_out, 0);
+    outuint(c_mix_out, underflowSample);
 #else
+    int outSamps;
     if(outUnderflow)
     {
 #pragma xta endpoint "out_underflow"

module_usb_audio/usb_buffer/usb_buffer.xc

@@ -180,7 +180,9 @@ void buffer(register chanend c_aud_out, register chanend c_aud_in,
     unsigned expected_fb = 0;
 #endif
 
+#if (NUM_USB_CHAN_OUT > 0)
     xc_ptr aud_from_host_buffer = 0;
+#endif
 
 #ifdef MIDI
     xc_ptr midi_from_host_buffer = array_to_xc_ptr(g_midi_from_host_buffer);