Hi Akshat,
Yes, you should wait for the BIST to complete before reading the results. If you read the REQBIST register immediately after entering BIST mode, the test may still be running, causing the registers to return invalid or default values. So after setting the BIST bit, insert a 15ms delay in your code before reading the results. Or you can poll the BIST bit until it clears (indicating BIST has completed).
BR, Tomas