core: fix Doxygen warning

libusbx/libusb/libusb.h:153: warning: no matching file member found for
libusb_cpu_to_le16()Possible candidates:
  static uint16_t libusb_cpu_to_le16(const uint16_t x)

Thanks to Serhat Sevki Dincer for the bug report
http://sourceforge.net/mailarchive/message.php?msg_id=31719691