hal: Fix 64 bit compilation error for usbaudio

- Fix 64 bit compilation error int to pointer
  seen in usbaudio file

Change-Id: I7b7a3cb097c0c645514fd4de36146ccc49c24896
diff --git a/hal/audio_extn/usb.c b/hal/audio_extn/usb.c
index 0240d7c..bc83709 100644
--- a/hal/audio_extn/usb.c
+++ b/hal/audio_extn/usb.c
@@ -140,7 +140,7 @@
 
     if (next_sr_string == NULL) {
         ALOGE("%s: get_numof_rates: could not find rates string", __func__);
-        return (int)NULL;
+        return 0;
     }
 
     for (i = 1; next_sr_string != NULL; i++) {