ACPI: property: Use acpi_dev_parent()

After introducing acpi_dev_parent() in commit 62fcb99b ("ACPI: Drop
parent field from struct acpi_device"), it is better to use it instead
of accessing the dev.parent field in struct acpi_device directly.

Modify acpi_node_get_parent() accordingly.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>