ASoC: ti: davinci-evm: 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>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201029082513.28233-1-vulab@iscas.ac.cn
Signed-off-by: Mark Brown <broonie@kernel.org>