leds: rt8515: Put fwnode in any case during ->probe()

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

Fixes: e1c6edcb

 ("leds: rt8515: Add Richtek RT8515 LED driver")
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>