Use defined constant for WEBSOCKET_OPCODE_CONNECTION_CLOSE test.

src/civetweb.c

@@ -4370,7 +4370,7 @@ static void read_websocket(struct mg_connection *conn)
             // or "connection close" opcode received.
             if ((conn->ctx->callbacks.websocket_data != NULL &&
                  !conn->ctx->callbacks.websocket_data(conn, buf[0], data, data_len)) ||
-                (buf[0] & 0xf) == 8) {  // Opcode == 8, connection close
+                (buf[0] & 0xf) == WEBSOCKET_OPCODE_CONNECTION_CLOSE) {  // Opcode == 8, connection close
                 break;
             }