Add USB audio dynamic channel mask to Nexus 10

Change-Id: I961f5ef809a939bda0534c24990ab567125996cd
diff --git a/audio_policy.conf b/audio_policy.conf
index 3063575..00f575f 100644
--- a/audio_policy.conf
+++ b/audio_policy.conf
@@ -73,7 +73,7 @@
       }
       usb_device {
         sampling_rates dynamic
-        channel_masks AUDIO_CHANNEL_OUT_STEREO
+        channel_masks dynamic
         formats dynamic
         devices AUDIO_DEVICE_OUT_USB_DEVICE
       }