ASoC: imx-audmix: Add NULL check in imx_audmix_probe

ASoC: imx-audmix: Add NULL check in imx_audmix_probe

devm_kasprintf() can return a NULL pointer on failure,but this
returned value in imx_audmix_probe() is not checked.
Add NULL check in imx_audmix_probe(), to handle kernel NULL
pointer dereference error.

Fixes: 05d996e11348 ("ASoC: imx-audmix: Split capture device for audmix")
Signed-off-by: Charles Han <hanchunchao@inspur.com>
Link: https://patch.msgid.link/20241118084553.4195-1-hanchunchao@inspur.com
Signed-off-by: Mark Brown <broonie@kernel.org>