ACPI / dock: simplify dock_create_acpi_device()

The return value of dock_create_acpi_device() is not used at all,
so change its signature to return void and simplify the
implementation of it.

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