ASoC: amd: remove set but not used variable 'dma_buffer'

Fixes gcc '-Wunused-but-set-variable' warning:

sound/soc/amd/raven/acp3x-pcm-dma.c: In function 'acp3x_dma_hw_params':
sound/soc/amd/raven/acp3x-pcm-dma.c:333:25: warning:
variable 'dma_buffer' set but not used [-Wunused-but-set-variable]

It never used since introduction in commit
8de1b5ed0337 ("ASoC: amd: add acp3x system resume pm op")

Signed-off-by: YueHaibing <[e-mail]>
Signed-off-by: Mark Brown <[e-mail]>

sound/soc/amd/raven/acp3x-pcm-dma.c

···
 {
 	int status;
 	u64 size;
-	struct snd_dma_buffer *dma_buffer;
 	struct page *pg;
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	struct i2s_stream_instance *rtd = runtime->private_data;
···
 	if (!rtd)
 		return -EINVAL;
 
-	dma_buffer = &substream->dma_buffer;
 	size = params_buffer_bytes(params);
 	status = snd_pcm_lib_malloc_pages(substream, size);
 	if (status < 0)