Bluetooth: Host: Fix warning from the logging subsystem

Cast `dhkey` to `void*` to avoid a warning from the logging subsystem:
```
<wrn> cbprintf_package: (unsigned) char * used for %p argument.
It's recommended to cast it to void * because it may cause misbehavior
in certain configurations
```

Signed-off-by: Herman Berget <herman.berget@nordicsemi.no>