Apollo firmware will keep the USB switch handed over to the FPGA as
long as the gateware keeps advertising its usage through the FPGA_ADV
pin. When these messages stop arriving, Apollo will take over the port.
If the advertisement resumes, the port will not be handed off to the
FPGA again until an "honor FPGA_ADV" (0xc2) vendor request arrives.

When this is added as a submodule to a design, an advertisement message
is sent periodically to Apollo. Apollo will take over the port when
these announcements are interrupted or when the PROGRAM button is
pressed.

An optional request handler is added: REQUEST_APOLLO_ADV_STOP (0xf0).
It returns the CONTROL port to Apollo by stopping announcements.

The host tools can request gateware to hand off the USB port to Apollo
if the necessary request is available (REQUEST_APOLLO_ADV_STOP).

Simplify the control flow and add error messages.

Requests handoff to Apollo using an existing device handle.

Co-authored-by: Martin Ling <[email protected]>