ASoC: Intel: fix warning reported by static check tool smatch

The smatch tool report warning:
  ...
    CHECK   sound/soc/intel/sst-haswell-pcm.c
  sound/soc/intel/sst-haswell-pcm.c:1110 hsw_pcm_probe() error: buffer overflow\
'hsw_dais' 4 <= 4
  sound/soc/intel/sst-haswell-pcm.c:1112 hsw_pcm_probe() error: buffer overflow\
'hsw_dais' 4 <= 4
  ...
fix it by use its own struct member for post-process module, rather than sharing
unused pcm member.

Signed-off-by: Lu, Han <han.lu@intel.com>
Acked-by: Jie Yang <yang.jie@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>