device property: fwnode_property_read_string_array() returns nr of strings

Functionally fwnode_property_read_string_array() should match
of_property_read_string_array() and work as a drop-in substitute for the
latter. of_property_read_string_array() returns the number of strings read
if the target string pointer array is non-NULL. Make
fwnode_property_read_string_array() do the same.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>