ASoC: Intel: sst: remove redundant variable dma_dev_name

The pointer dma_dev_name is assigned but never read, it is redundant
and can therefore be removed.

Cleans up clang warning:
sound/soc/intel/common/sst-firmware.c:288:3: warning: Value stored to
'dma_dev_name' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>