iio: adc: ti-ads1015: add missing of_node_put() in ads1015_get_channels_config_of()

When terminating for_each_child_of_node() iteration with
break or return, of_node_put() should be used to prevent
stale device node references from being left behind.

This is detected by Coccinelle semantic patch.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>