usb_dev: Better document how to connect the driver to firmware
usb_dev: Better document how to connect the driver to firmware

While the existing documentation was acceptable, it did not explain how to
connect the control request handler to firmware, leaving it as an exercise
to the reader. Add this missing documentation.

Signed-off-by: Alexandru Gagniuc <cf8ce030f4bdd6ce49a1b687c7c3ccfa8b34ced2@gmail.com>

mrnuke/qiprog