A lot of this code comes from folks who are a lot smarter than I am!
The original SPI code comes from the talented Anatolij Gustschin:
https://lkml.org/lkml/2018/11/27/1221
Then, additional patches to add GPIO support were written by Ben Maddocks:
https://github.com/bm16ton/ft2232-mpsse-i2c-spi-kern-drivers
My code is based upon Ben Maddocks', with stuff we didn't need (like polled interrupts)
removed for maintainability's sake and better support for our FT2232 chip (like accessing
both MPSSEs!)