ASoC: imx-audmix: Add NULL check in imx_audmix_probe

In the Linux kernel, the following vulnerability has been resolved: 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.