software node: simplify property_entry_read_string_array()

There is no need to treat string arrays and single strings separately, we can go
exclusively by the element length in relation to data type size.

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