Bugs in saHpiSensorThresholdsSet_T19

It is skipped when sensor thresholds are not writable: it
should be the contrary.

Hysteresis should be set to a negative value: this is
what is forbidden by the spec and not the setting of
NegThdHysteresis.

It should only be performed on sensors that support
hysteresis and have a reading format that allows using
negative values.

The attached patch fixes all these issues.