staging: iio: ad7192: Use devm_* APIs and fix a memory leak

devm_* APIs are device managed and make code simpler.

The memory leak was a lack of freeing the iio_dev structure in the
remove function.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>