VALUE
method_initialize (VALUE dl)
{
  int rc;

  rc = usb_init (NUM2INT(dl));
  if (rc)
    return Qnil;
  rc = usb_open_dev ();
  if (rc)
    return Qnil;
}

rb_define_method(HTLLUSB, "initialize", method_initialize, 1);

@dev = HTLLUSB.new 2

leads to:
usb_iface.rb:7:in `initialize': can't convert HTLLUSB into Integer (TypeError)