iio: adc: ti-adc081c: Use devm_* APIs

devm_* APIs are device managed and make code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>