hwmon: (pmbus/isl68137) Fix child node reference leak on early return

hwmon: (pmbus/isl68137) Fix child node reference leak on early return

In the case of an early return, the reference to the child node needs
to be released.

Use for_each_child_of_node_scoped to fix the issue.

Fixes: 3996187f80a0e ("hwmon: (pmbus/isl68137) add support for voltage divider on Vout")
Signed-off-by: Erick Karanja <karanja99erick@gmail.com>
Link: https://lore.kernel.org/r/20251012181249.359401-1-karanja99erick@gmail.com
[groeck: Updated subject/description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>