remove border router from pan size calculation

According to specification border router is not counted to the PAN size

Author: Mika Tervonen

source/6LoWPAN/ws/ws_bbr_api.c

@@ -507,6 +507,10 @@ uint16_t ws_bbr_pan_size(protocol_interface_info_entry_t *cur)
     }
 
     rpl_control_get_instance_dao_target_count(cur->rpl_domain, RPL_INSTANCE_ID, NULL, prefix_ptr, &result);
+    if (result > 0) {
+        // remove the Border router from the PAN size
+        result--;
+    }
     return result;
 }