software node: Use kobject name when finding child nodes by name

Using the kobject name of the node instead of a device
property "name" in software_node_get_named_child_node().

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>