ALSA: x86: Drop unused hw_silence field

It's nowhere used.  Let's drop it.
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/x86/intel_hdmi_audio.c

@@ -1696,7 +1696,6 @@ int hdmi_audio_probe(struct platform_device *devptr,
 	pm_runtime_set_active(intelhaddata->dev);
 	pm_runtime_enable(intelhaddata->dev);
 
-	intelhaddata->hw_silence = 1;
 	*had_ret = intelhaddata;
 
 	return 0;

sound/x86/intel_hdmi_audio.h

@@ -118,7 +118,6 @@ struct had_pvt_data {
  * @kctl: holds kctl ptrs used for channel map
  * @chmap: holds channel map info
  * @audio_reg_base: hdmi audio register base offset
- * @hw_silence: flag indicates SoC support for HW silence/Keep alive
  * @underrun_count: PCM stream underrun counter
  */
 struct snd_intelhad {
@@ -142,7 +141,6 @@ struct snd_intelhad {
 	struct snd_pcm_chmap *chmap;
 	unsigned int	*audio_reg_base;
 	unsigned int	audio_cfg_offset;
-	bool		hw_silence;
 	int underrun_count;
 };