Users are supposed to clear SUDAVIRQ first and then read 8 data bytes.
This is because CPU checks these 8 data bytes to decide the final status of particular USB control transfer. Status could be stall (error)/ACK (success).
Until this status is decided, SIE continuously sends NAK handshake to host for this CONTROL transfer stage.
Since host would already be getting NAK, it will keep on retrying till the device is ready.