iio: at91_adc: Add missing CONFIG_OF macro

The data structure is required only when DT is enabled.
Hence compile it conditionally.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>