ASoC: hdac_hdmi: Fix possible memory leak in hw_params

It's possible for hw_params to be called two times. So add NULL
check to prevent memory leak.

Signed-off-by: Subhransu S. Prusty <subhransu.s.prusty@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>