leds: led-core: Update fwnode with device_set_node

Update a newly created device's fwnode and of_node pointers using the
recently added device_set_node helper. This keeps some firmware node
specifics out of led-class and should help tracking future changes
regarding device firmware node updates.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Pavel Machek <pavel@ucw.cz>