drm/nouveau/temp: Fix signed/unsigned int logic

Many (all?) of the coefficients related to calculating the
correct temperature are signed integers

This patch correcly parses and stores those values
It also ensures that the default offset is 0 (previously 1)

Affected cards - the original nv50 and the nv40 family

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>