ALSA: hda: ext: make snd_hdac_ext_stream_init() static

ALSA: hda: ext: make snd_hdac_ext_stream_init() static

There are no external users of this helper, move to static and remove
sympol export. No functionality change.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20220919121041.43463-4-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>