sys/transceiver: add transceiver_unregister and refactor transceiver_register

I'd like
```c
int result = 0;
int state = disableIRQ();
for (...) {
    if (...) {
        ...;
        result = 1;
        break;
    }
}
restoreIRQ(state);
return result;
```
better. :)