software node: clean up property_copy_string_array()

Because property_copy_string_array() stores the newly allocated pointer in the
destination property, we have an awkward code in property_entry_copy_data()
where we fetch the new pointer from dst.

Let's change property_copy_string_array() to return pointer and rely on the
common path in property_entry_copy_data() to store it in destination structure.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>