leds: lp50xx: Put fwnode in error case during ->probe()

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

OTOH, the successful iteration will drop reference count under the hood, no need
to do it twice.

Fixes: 242b8117 ("leds: lp50xx: Add the LP50XX family of the RGB LED driver")
Cc: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>