atusb/fw/: SET_INTERFACE can now select among interfaces (changes user_setup)

- ep0.c (ep0_init), usb/dfu.c (dfu_init): set user_setups[0] instead
  of user_setup
- usb/atu2.c (usb_poll): reset user_setup on bus reset
- usb/usb.h (user_setups), usb/usb.c (user_setups): array of
  interface-specific setup functions
- usb/usb.c (handle_setup): in SET_INTERFACE, select setup function
  from user_setups according to interface
- usb/usb.c (handle_setup): if user_setup is not set (e.g., the
  optional SET_INTERFACE was never issued), fall back to user_setups[0]

atusb/fw/ep0.c

@@ -209,5 +209,5 @@ static int my_setup(const struct setup_request *setup)
 
 void ep0_init(void)
 {
-	user_setup = my_setup;
+	user_setups[0] = my_setup;
 }