USBDFU should be updated to not use deprecated `SETUP_EP0_BUF(0);` call.

[purdeaandrei]

After PR #18 is merged, the USBDFU code should be updated as well.
Exactly how I'm not sure, since it's not trivial.

The simplest non-functional change would be:

diff --git a/firmware/library/usbdfu.c b/firmware/library/usbdfu.c
index 38ca144..2fc9178 100644
--- a/firmware/library/usbdfu.c
+++ b/firmware/library/usbdfu.c
@@ -93,14 +93,16 @@ bool usb_dfu_setup(usb_dfu_iface_state_t *dfu, __xdata struct usb_req_setup *req
         dfu->length   = req->wLength;
         dfu->pending  = true;
         dfu->sync     = false;
-        SETUP_EP0_BUF(0);
+        SETUP_EP0_OUT_BUF();
+        ACK_EP0(); // TODO: this should probably be refactored to ack the buffer only after it's been processed?
         return true;
       } else if(dfu->state == USB_DFU_STATE_dfuDNLOAD_IDLE && req->wLength > 0) {
         dfu->state    = USB_DFU_STATE_dfuDNLOAD_SYNC;
         dfu->length   = req->wLength;
         dfu->pending  = true;
         dfu->sync     = false;
-        SETUP_EP0_BUF(0);
+        SETUP_EP0_OUT_BUF();
+        ACK_EP0(); // TODO: this should probably be refactored to ack the buffer only after it's been processed?
         return true;
       } else if(dfu->state == USB_DFU_STATE_dfuDNLOAD_IDLE) {
         dfu->state    = USB_DFU_STATE_dfuMANIFEST_SYNC;

However reading the code, it seems like USB_DFU_STATE_dfuDNLOAD_SYNC state exists specifically to work around the problem introduced by SETUP_EP0_BUF(0);, so I think the right implementation is to remove the USB_DFU_STATE_dfuDNLOAD_SYNC state completely.

Since the correct solution is not necessarily trivial, I'm not submitting a PR for this.

[purdeaandrei]

I have read the DFU spec, and obviously what I said here was wrong:

However reading the code, it seems like USB_DFU_STATE_dfuDNLOAD_SYNC state exists specifically to work around the problem introduced by SETUP_EP0_BUF(0);

I have implemented a correct fix in PR 18, and tested it too, so closing this issue