Don't set the A2DP sink data callback when registering A2DP source

The p_sink_data_cback pointer should be NULL when registering
A2DP source with BTA_AvRegister()

Change-Id: I1a709c4e0d3cfe6faf0368a013b391c6e6a53a82
diff --git a/system/btif/src/btif_av.cc b/system/btif/src/btif_av.cc
index da30e3a..1287e99 100644
--- a/system/btif/src/btif_av.cc
+++ b/system/btif/src/btif_av.cc
@@ -1560,7 +1560,7 @@
          BTA_AvEnable(BTA_SEC_AUTHENTICATE, (BTA_AV_FEAT_RCTG | BTA_AV_FEAT_NO_SCO_SSPD),
                       bte_av_callback);
 #endif
-         BTA_AvRegister(BTA_AV_CHNL_AUDIO, BTIF_AV_SERVICE_NAME, 0, bte_av_sink_media_callback,
+         BTA_AvRegister(BTA_AV_CHNL_AUDIO, BTIF_AV_SERVICE_NAME, 0, NULL,
                         UUID_SERVCLASS_AUDIO_SOURCE);
      }
      else {