iio: adc: stm32-adc: Remove redundant null check before clk_prepare_enable/clk_disable_unprepare

Because clk_prepare_enable() and clk_disable_unprepare() already checked
NULL clock parameter, so the additional checks are unnecessary, just
remove them.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Acked-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20201218093512.871-1-vulab@iscas.ac.cn


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