ASoC: Intel: boards: hda_dsp_common: use NULL pointer assignment, not 0

Fix Sparse warning:
hda_dsp_common.c:66:37: warning: Using plain integer as NULL pointer

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200102195952.9465-2-pierre-louis.bossart@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>