ASoC: fsl: fsl_dma: remove unused variable

ASoC: fsl: fsl_dma: remove unused variable

cppcheck warning:

sound/soc/fsl/fsl_dma.c:411:10: style: Variable 'channel' is assigned
a value that is never used. [unreadVariable]

 channel = substream->stream == SNDRV_PCM_STREAM_PLAYBACK ? 0 : 1;
         ^

Removing this line shows the variable isn't needed any longer so
remove declaration as well.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210219232937.6440-3-pierre-louis.bossart@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>