ASoC: Intel: Skylake: remove useless cast

Detected with Coccinelle

sound/soc/intel/skylake/skl-topology.c:3106:16-20: WARNING: casting
value returned by memory allocation function to (char *) is useless.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>