RP2 SPI: Instantiating with spi=None does not behave sensibly.

The constructor accepts `None` as an arg to `miso`, but still instantiates the default pin. 

The clear intent is to create an output-only interface. Either it should reject the arg or (preferably) it should not instantiate a pin. Current behaviour can lead to unexpected pin conflicts.

This should also apply to `mosi`.