Bluetooth: Send Error for Invalid AT Command.

Some Headsets send HFP 1.6 cmds even when it is not
supported by DUT.Send error response in those cases.

bug 11450717

Change-Id: If7745959c00fd49bd737b41a49adba9e726aa5c7
diff --git a/bta/ag/bta_ag_cmd.c b/bta/ag/bta_ag_cmd.c
index 0839cbb..c035cf3 100644
--- a/bta/ag/bta_ag_cmd.c
+++ b/bta/ag/bta_ag_cmd.c
@@ -1246,6 +1246,7 @@
 #endif
 
         default:
+            bta_ag_send_error(p_scb, BTA_AG_ERR_OP_NOT_SUPPORTED);
             break;
     }