drm/nouveau: constify pointers to hwmon_channel_info

Statically allocated array of pointers to hwmon_channel_info can be made
const for safety.

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230511175443.281970-1-krzysztof.kozlowski@linaro.org