ACPI / fan: Make struct dev_pm_ops const

Silence the following checkpatch warning:
WARNING: struct dev_pm_ops should normally be const.

Signed-off-by: Kaiyen Chang <kaiyen.chang@intel.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>