leds: lt3593: do not rewrite .of_node of new LED device to wrong value

The devm_led_classdev_register_ext is given init_data with fwnode set,
so the LED core sets .of_node of the new LED classdev correctly.

Do not rewrite this value to parent of_node.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Pavel Machek <pavel@ucw.cz>