iio: adc: imx25-gcq: Fix leak of device_node in mx25_gcq_setup_cfgs()

Leaving for_each_child_of_node loop we should release child device node,
if it is not stored for future use.

Found by Linux Driver Verification project (linuxtesting.org).

JC: I'm not sending this as a quick fix as it's been wrong for years,
but good to pick up for stable after the merge window.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Fixes: 6df2e98c ("iio: adc: Add imx25-gcq ADC driver")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>