iio: adc: stm32-dfsdm: Remove redundant null check before clk_disable_unprepare

ecause clk_disable_unprepare() already checked NULL clock parameter,
so the additional check is unnecessary, just remove it.

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20201231085322.24398-1-vulab@iscas.ac.cn


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