ASoC: soc-generic-dmaengine-pcm: cleanup cppcheck warning at dmaengine_pcm_new()

This patch cleanups below cppcheck warning.

sound/soc/soc-generic-dmaengine-pcm.c:233:28: style: The scope of the variable 'substream' can be reduced. [variableScope]
 struct snd_pcm_substream *substream;
                           ^

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/871r6u0yzs.wl-kuninori.morimoto.gx@renesas.com


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