ASoC: Intel: sst: remove unused 'msg_high'

In process_fw_async_msg(), a variable 'msg_high' is initialized but
not used. So remove it.

sound/soc/intel/atom/sst/sst_ipc.c: In function ‘process_fw_async_msg’:
sound/soc/intel/atom/sst/sst_ipc.c:263:24: warning: variable ‘msg_high’ set but not used [-Wunused-but-set-variable]
  union ipc_header_high msg_high;

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>