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
8de1b5ed ("ASoC: amd: add acp3x system resume pm op")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>