drm/nouveau/therm: do not make assumptions on temperature

In nouveau_therm_sensor_event, temperature is stored as an uint8_t
even though the original interface returns an int.

This change should make it more obvious when the sensor is either
very-ill-calibrated or when we selected the wrong sensor style
on the nv40 family.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>