device property: check fwnode type in to_of_node()

Potentially one of platform can support both ACPI and OF. In that case when we
call to_of_node() for non-OF fwnode types we will get non-NULL result, which is
wrong. Check for the type and return a correspondent result.

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>