VolumeControl: Early return in unregisterCallback

replaced:
```
if (foo) {
  /*
  * long code
  */
}
```
by
```
if (!bar) {
  return;
}
/*
* long code
*/
```

To win some indent spaces

Bug: 324492914
Test: m Bluetooth | Existing test
Flag: Exempt, mechanical no-op
Change-Id: Ib735d03f1ea14547aae9213b6f7abc3362ff1338