ASoC: es8328: Remove redundant null check before clk_disable_unprepare

Because 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/20210108085834.7168-1-vulab@iscas.ac.cn
Signed-off-by: Mark Brown <broonie@kernel.org>