nonce function does not generate a nonce

[nonce](https://github.com/adafruit/Adafruit_CircuitPython_ATECC/blob/a39bdd64be6b09acc272e59241115cc0074038ff/adafruit_atecc/adafruit_atecc.py#L275) function is not generating a unique nonce.

This may be due to #21

```
...
print(atecc.nonce(data=bytearray(b'\x03\x1e\xdd}Ae\x15\x93\xc5\xfe\\\x00o\xa5u+7\xfd\xdf\xf7')))
...
```

Output is the same every time:
```
bytearray(b'\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00\xff\xff\x00\x00')
```