busio.SPI locks GPIO12 even when it's not used on ESP8266

Trying to initialize DigitalInOut on pin GPIO12 results in:

```
>>> dc = digitalio.DigitalInOut(board.GPIO12)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Pin MTDI in use
```
Tested with CircuitPython 2.2.3.