leds: lm3692x: 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: 9a5c1c64 ("leds: lm3692x: Change DT calls to fwnode calls")
Cc: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>