Parity shouldn't be changing data bits

This fixes a bug where the data bits parameter was being set to 7 for even and odd parity, even when it was specifically set to 8 by the user. For example, your data would have been mangled if you requested the following:

```javascript
var serial = new SerialPort('/dev/ttyUSB0', {parity: 'even', dataBits: 8});
```