iio: exynos_adc: fix wrong structure extration in suspend and resume

The exynos_adc device structure was wrongly extracted from the dev*
correcting the same.

Using the regular conversion of
struct device* -> struct platform_device* -> struct exynos_adc* seems wrong.
Instead we should be doing
struct device* -> struct iio_dev* -> struct exynos_adc*

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