hal: add USB capture backend configuration support

Add USB capture BE configuration support and update the parsing
function to retrieve USB device capability.

Change-Id: I3dc7d289b3fcc96d522a2df3b8168f546737c036
CRs-Fixed: 1050510 1054988 1050666 1058845
diff --git a/hal/msm8974/platform.h b/hal/msm8974/platform.h
index 9b8c01b..bcf5d93 100644
--- a/hal/msm8974/platform.h
+++ b/hal/msm8974/platform.h
@@ -201,7 +201,6 @@
 #define OUTPUT_SAMPLING_RATE_44100      44100
 #define OUTPUT_SAMPLING_RATE_DSD64       176400
 #define OUTPUT_SAMPLING_RATE_DSD128      352800
-#define MAX_CODEC_TX_BACKENDS           1
 enum {
     DEFAULT_CODEC_BACKEND,
     SLIMBUS_0_RX = DEFAULT_CODEC_BACKEND,
@@ -217,6 +216,13 @@
     MAX_CODEC_BACKENDS
 };
 
+enum {
+    DEFAULT_CODEC_TX_BACKEND,
+    SLIMBUS_0_TX = DEFAULT_CODEC_TX_BACKEND,
+    USB_AUDIO_TX_BACKEND,
+    MAX_CODEC_TX_BACKENDS
+};
+
 #define AUDIO_PARAMETER_KEY_NATIVE_AUDIO "audio.nat.codec.enabled"
 #define AUDIO_PARAMETER_KEY_NATIVE_AUDIO_MODE "native_audio_mode"