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
diff --git a/firmware/ice40-riscv/icE1usb/Makefile b/firmware/ice40-riscv/icE1usb/Makefile
index 25c65d1..ed765f6 100644
--- a/firmware/ice40-riscv/icE1usb/Makefile
+++ b/firmware/ice40-riscv/icE1usb/Makefile
@@ -56,6 +56,7 @@
 	usb_dev.h \
 	usb_e1.h \
 	usb_gps.h \
+	usb_gpsdo.h \
 	usb_str_app.gen.h \
 	$(NULL)
 
@@ -69,6 +70,7 @@
 	usb_dev.c \
 	usb_e1.c \
 	usb_gps.c \
+	usb_gpsdo.c \
 	$(NULL)