drm/nouveau/therm: fix spelling mistake on array thresolds

Array thresolds should be named thresholds, rename it. Also make it static
static const char * const

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>