Ignore 0x8000, 0x8000 reports since they're bad

[fhunleth]

If the SHT4X returns a raw temperature of 0x8000 and a raw humidity of 0x8000, Sensirion is saying that the sensor might be affected by ESD and should not be trusted. On the sensor that suffers from the 0x8000 problem, it rarely reports 0x8001 for the temperature, so that should be marked as :unusable as well.

The 0x8000 temperature corresponds to around 40C, so it's not obviously bad if you don't know where the sensor is located. Reading a 0x8000 for the humidity at the same time seems to be the key in detecting the condition.