ASoC: imx-audmix: check return value of devm_kasprintf()

devm_kasprintf() returns a pointer to dynamically allocated memory.
Pointer could be NULL in case allocation fails. Check pointer validity.
Identified with coccinelle (kmerr.cocci script).

Fixes: b86ef536 ("ASoC: fsl: Add Audio Mixer machine driver")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20230614121509.443926-1-claudiu.beznea@microchip.com


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