ftdi_eeprom: added --device option to specify FTDI device

ftdi_eeprom: added --device option to specify FTDI device

 * previously, the device could only be selected using the "vendor_id",
   "product_id" and "default_pid" config file options.
   This did not guarantee that a device could be uniquely identified
   (e.g. there could be multiple devices with the same VID/PID).
   Also this severely limited the possibilities of changing a device's
   VID/PID using ftdi_eeprom - this only worked if the device happened
   to have FTDI's VID 0x0403 and a PID equal to "default_pid".
 * If the --device option is omitted, ftdi_eeprom defaults to the
   old behaviour of using the config file options.
 * The order of arguments is insignificant. If multiple
   'command' options (--read-eeprom, --erase-eeprom, --flash-eeprom)
   are given, only the last one will determine the action.