ACPI: properties: Return _DSD hierarchical extension (data) sub-nodes correctly

The recently merged patch "ACPI: Prepare for constifying
acpi_get_next_subnode() fwnode argument" was part of a patchset
constifying the fwnode arguments across the fwnode property API. The
purpose of the patch was to allow returning non-const fwnodes from a data
structure the root of which is const.

Unfortunately the patch introduced the functionality, in particular when
starting parsed from an ACPI device node, the hierarchical data extension
nodes would not be enumerated.

Restore the old behaviour while still retaining constness properties of
the patch.

Fixes: 01c1da28 "ACPI: Prepare for constifying acpi_get_next_subnode() fwnode argument"
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>