SMIBHID SCD30 get_formatted_reading method crashes on NaN

[sjefferson99]

The SCD30 module can return NaN when the sensor does not have a value ready to send. This is either due to automatic readings not enabled or not yet running (has a warm up time of a few seconds) or because the data was polled more frequently than the auto poll frequency as the read method clears the reading from the sensor buffer.

Fix the method to gracefully handle NaN and return a value to that effect.