ALSA: hda - Add CONFIG_SND_HDA_POWER_SAVE to stac_vrefout_set()

In commit 45eebda7

, it add new function stac_vrefout_set, but it
is only used in code between CONFIG_SND_HDA_POWER_SAVE macro, so
add the macro to avoid such warning:

  sound/pci/hda/patch_sigmatel.c:676:12: warning: 'stac_vrefout_set' defined but not used

Signed-off-by: Wang Shaoyan <wangshaoyan.pt@taobao.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>