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

sound/soc/intel/haswell/sst-haswell-ipc.c: In function 'hsw_stream_message':
sound/soc/intel/haswell/sst-haswell-ipc.c:669:29: warning: variable 'stage_type' set but not used [-Wunused-but-set-variable]

It is never used since introduction in
commit ba57f682 ("ASoC: Intel: create haswell folder and move haswell platform files in")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>