leds: lt3593: Put fwnode in any case during ->probe()

device_get_next_child_node() bumps a reference counting of a returned variable.
We have to balance it whenever we return to the caller.

Fixes: 8cd7d6da

 ("leds: lt3593: Add device tree probing glue")
Cc: Daniel Mack <daniel@zonque.org>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>