Reduce log level when temp = 0

When a sensor temperature is equal to 0, currently the log message
level is warning. If some sensor is bad it will fill log files,
so reduce the log level to debug.
trip temp and hyst less than 0

If there is a buggy thermal sysfs entry, which is less than 0,
use defaults.
Merge pull request #25 from 01org/development_0

Development 0