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/usb/atu2.c

@@ -190,6 +190,7 @@ void usb_poll(void)
 	if (flags & (1 << EORSTI)) {
 		if (user_reset)
 			user_reset();
+		user_setup = NULL;
 		ep_init();
 		UDINT &= ~(1 << EORSTI);
 	}