DHT22 negative temperatures are not handled correctly.

[jerryneedell]

The DHT22 sensor returs a 16 bit temperature with bits 0-14 containing the magnitude and the MSB indicating the sign. The driver as written incorrectly treat the value as a twos-complement signed integer and gives incorrect results below 0 C .
I put the sensor in a freezer.
the dht_simpletest converts to F so this example shows the error as the temperature crosses 32 F

Temp: 32.9 F Humidity: 23.4% 
Temp: 32.7 F Humidity: 23.5% 
Temp: 32.5 F Humidity: 23.6% 
Temp: 32.4 F Humidity: 23.7% 
Temp: 32.2 F Humidity: 23.8% 
Temp: 32.0 F Humidity: 23.8% 
Temp: 5930.4 F Humidity: 24.0% 
Temp: 5930.4 F Humidity: 24.1% 
Temp: 5930.5 F Humidity: 24.2% 
Temp: 5930.7 F Humidity: 24.3% 
Temp: 5930.9 F Humidity: 24.4% 
Temp: 5931.1 F Humidity: 24.5% 
Temp: 5931.3 F Humidity: 24.5% 
Temp: 5931.4 F Humidity: 24.7% 
Temp: 5931.6 F Humidity: 24.8% 
Temp: 5931.6 F Humidity: 25.1% 
Temp: 5931.8 F Humidity: 25.7% 
Temp: 5931.6 F Humidity: 26.3% 
Temp: 5931.6 F Humidity: 26.9% 
Temp: 5931.6 F Humidity: 27.6% 
Temp: 5931.3 F Humidity: 28.7% 
Temp: 5931.1 F Humidity: 29.2% 
Temp: 5930.9 F Humidity: 29.7% 
Temp: 5930.5 F Humidity: 30.2% 
Temp: 5930.4 F Humidity: 30.9% 
Temp: 32.0 F Humidity: 31.4% 
Temp: 32.5 F Humidity: 32.7% 
Temp: 32.9 F Humidity: 33.3%

Note - the DHT 11 appear to only measure down to 1 C - so there is no problem there.

[jerryneedell]

fixed by PR #10