device property: Use fwnode_graph_for_each_endpoint() macro

Now that we have fwnode_graph_for_each_endpoint() macro, use it instead of
calling fwnode_graph_get_next_endpoint() directly. It manages the iterator
variable for the user without manual intervention.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>