ASoC: sigmadsp: switch to use kmemdup_nul() helper

Use kmemdup_nul() helper instead of open-coding to
simplify the code.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220825123525.1845695-1-yangyingliang@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>