ACPI / dock: drop redundant spin lock in dock station object

All dock station objects are created during initialization and
don't change at runtime, so drop the redundant spin lock from
struct dock_station.

[rjw: Changelog]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>