icE1usb: Move GPS-DO USB control to separate USB interface

doing so significantly simplifies the development of a Linux kernel
driver, as the GPS-DO only exists once (not twice, like the per-E1-line
interface), and Linux kernel USB drivers typically are for an interface.

There is an option to write a usb_device_driver, but doing so will
exclude the per-interface drivers from still being probed in their usual
fashion.

While we introduce this new USB Interface for the GPS-DO, we also
move the related control endpoint requests from the device level to the
interface level.

Finally, some naming inconsistency between "enum
ice1usb_gpsdo_antenna_state" vs. the member name antenna_status is
resolved.

Change-Id: Icd6555a14896c38626fb147b78af44ff719f2254
---
M firmware/ice40-riscv/icE1usb/Makefile
M firmware/ice40-riscv/icE1usb/fw_app.c
M firmware/ice40-riscv/icE1usb/gpsdo.c
M firmware/ice40-riscv/icE1usb/ice1usb_proto.h
M firmware/ice40-riscv/icE1usb/usb_desc_app.c
M firmware/ice40-riscv/icE1usb/usb_desc_ids.h
M firmware/ice40-riscv/icE1usb/usb_dev.c
A firmware/ice40-riscv/icE1usb/usb_gpsdo.c
A firmware/ice40-riscv/icE1usb/usb_gpsdo.h
M firmware/ice40-riscv/icE1usb/usb_str_app.txt
10 files changed, 171 insertions(+), 81 deletions(-)