drm/nouveau/fan: fix selection of fan speed when fan->get returns an error

fan->get returns int, but we write it to unsigned variable, and then check
whether it's >= 0 (it always is)

Found by smatch:
drivers/gpu/drm/nouveau/core/subdev/therm/fan.c:61 nouveau_fan_update() warn: always true condition '(duty >= 0) => (0-u32max >= 0)'

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>