Replace eeprom->rs232_inversion with eeprom->invert.

Replace eeprom->rs232_inversion with eeprom->invert.

Summary:
 * The field eeprom->rs232_inversion reads and writes to the same
   location as eeprom->invert already does.
 * The configuration values invert_*, as read by ftdi_eeprom, are
   working ootb with FT230X chips. No configuration values were
   available for rs232_inversion.

I tested this commit with FT231X which identifies itself as VID 0x0403
and PID 0x6015.