ASoC: sh: siu_pcm: remove useless assignment

ASoC: sh: siu_pcm: remove useless assignment

cppcheck warning:

sound/soc/sh/siu_pcm.c:375:5: style: Redundant initialization for
'rt'. The initialized value is overwritten before it is
read. [redundantInitialization]

 rt = siu_stream->substream->runtime;
    ^
sound/soc/sh/siu_pcm.c:366:30: note: rt is initialized
 struct snd_pcm_runtime  *rt = ss->runtime;
                             ^
sound/soc/sh/siu_pcm.c:375:5: note: rt is overwritten
 rt = siu_stream->substream->runtime;
    ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210219231635.5749-8-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>