iio: frequency: adf4350: Use devm_* APIs

devm_* APIs are device managed and make code simpler.
This also takes care of missing clk_put function calls implicitly.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>