device property: fallback to pset when gettng one string

The one string as an equivalent to an array of one element. Allow user to read
one string as a plain string.

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