Flush queues on control endpoint when a new SETUP is received.

On macOS 12.6 running libusb 1.0.26, the host was seen to sometimes
make an incomplete ClearFeature(ENDPOINT_HALT) request when
ClearPipeStallBothEnds was called from libusb_cancel_transfer.

The host would send the SETUP packet, and the firmware would
call usb_transfer_schedule_ack to acknowledge the upcoming IN
token. However, the host would then not send the IN token,
proceeding directly to the next SETUP.

Since an empty transfer was queued to schedule the ACK, the
firmware would leak one transfer from the free_transfers list.

After a few iterations of this, the firmware would run out of
free transfers and the next request would hang waiting for one.

Fix this by flushing the transfer queues for the control endpoint
when a new SETUP is received, since that token implicitly cancels
any previously ongoing request.

firmware/common/usb_request.c

@@ -60,6 +60,11 @@ static void usb_request(usb_endpoint_t* const endpoint, const usb_transfer_stage
 
 void usb_setup_complete(usb_endpoint_t* const endpoint)
 {
+	// If we've received a new SETUP token, then any previously
+	// in-progress request is now aborted.
+	usb_endpoint_flush(endpoint->in);
+	usb_endpoint_flush(endpoint->out);
+
 	usb_request(endpoint, USB_TRANSFER_STAGE_SETUP);
 }