staging: iio: spear_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: Stefan Roese <sr@denx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>