drm/nouveau/hwmon: Remove old code, add .write/.read operations

This patch removes old code related to the old api and transforms the
functions for the new api. It also adds the .write and .read operations.
Signed-off-by: Oscar Salvador <osalvador.vilardaga@gmail.com>
Reviewed-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>