"if_mode" as option not available yet

The SDRPlay has an option `if_mode`:

```
* IF Mode - IF frequency in kHz
       [key=if_mode, default=Zero-IF, type=string, options=(Zero-IF, 450kHz, 1620kHz, 2048kHz)]
```

I see in the modul code, using this option is not implemented yet.
Please add this option, it's important to have the possibility for use this.
Thanks.

Heiko
