iio: adc: ingenic-adc: Benefit from devm_clk_get_prepared() to simplify

Make use of devm_clk_get_prepared() to replace some code that effectively
open codes this new function.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220808204740.307667-4-u.kleine-koenig@pengutronix.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>