iio: exynos_adc: use indio_dev->dev structure to handle child nodes

Using pdev->dev with device_for_each_child() would iterate over all
of the children of the platform device and delete them.
Thus, causing crashes during module unload.

We should be using the indio_dev->dev structure for
registering/unregistering child nodes.

Reported-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Naveen Krishna Ch <ch.naveen@samsung.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>