ASoC: codecs: rt715-sdca: simplify set_stream

Using a dynamic allocation to store a single pointer is not very
efficient/useful.

Worse, the memory is released in the SoundWire stream.c file, but
still accessed in the DAI shutdown, leading to kmemleak reports.

And last the API requires the previous stream information to be
cleared when the argument is NULL.

Simplify the code to address all 3 problems.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Rander Wang <rander.wang@intel.com>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Link: https://lore.kernel.org/r/20230324014408.1677505-13-yung-chuan.liao@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>