How do I program a SETUP transfer with the MAX3420E?

The USB host uses a CONTROL transfer to send a SETUP packet to the MAX3420E along with eight bytes that serve as a USB "op-code." The MAX3420E stores these bytes in an 8-byte FIFO, and then asserts a Setup Data Available interrupt request. The SPI master responds by reading the eight SETUP bytes at register address R4 (SUDFIFO), interpreting the USB request from these bytes, and taking the appropriate action. When finished servicing the request, the SPI master sets a bit called ACKSTAT (ACK from acknowledge, STAT from STATUS stage) to tell the MAX3420E to acknowledge the status stage of the CONTROL transfer.