hal: Add USB audio via ADSP support

Add changes to support USB audio via ADSP tunnel
solution.

CRs-Fixed: 1019158
Change-Id: I95a9796d7cad71e09dedd2bcd451183cbd25ca71
diff --git a/hal/msm8916/platform.h b/hal/msm8916/platform.h
index aef905d..7ff9c46 100644
--- a/hal/msm8916/platform.h
+++ b/hal/msm8916/platform.h
@@ -211,6 +211,7 @@
     HEADPHONE_BACKEND,
     SLIMBUS_6_RX = HEADPHONE_BACKEND,
     HDMI_RX_BACKEND,
+    USB_AUDIO_RX_BACKEND,
     MAX_CODEC_BACKENDS
 };
 #define AUDIO_PARAMETER_KEY_NATIVE_AUDIO "audio.nat.codec.enabled"