As of the control IN EP, the device must return ZLP when
- The number of bytes to return is just the multiple of bMaxPacketSize
AND
- The number of bytes to return is shorter than requested in wLength field on the SetUp command.

The first condition means that the descriptor size equals to 8, 16, 24, 32....., as bMaxPacketSize is 8 in your case.

Now, we are still handling Control IN transfer.
Maybe you are confused with Control OUT transfer.

Tsuneo