hal: update to add BE channel configuration for USB capture

Update to add BE channel configuration for USB capture and more
BE configuration options for sample rate and channel number.

Change-Id: I1e0af7a19a67aecba5afae95ecc62eec4a2d650b
CRs-Fixed: 1071868
diff --git a/hal/platform_api.h b/hal/platform_api.h
index 7bd6756..cdb7c5c 100644
--- a/hal/platform_api.h
+++ b/hal/platform_api.h
@@ -24,6 +24,8 @@
 #define CODEC_BACKEND_DEFAULT_BIT_WIDTH 16
 #define CODEC_BACKEND_DEFAULT_SAMPLE_RATE 48000
 #define CODEC_BACKEND_DEFAULT_CHANNELS 2
+#define CODEC_BACKEND_DEFAULT_TX_CHANNELS 1
+
 
 enum {
     NATIVE_AUDIO_MODE_SRC = 1,