ALSA: hda - Fix wrong volumes in AD1988 auto-probe mode

Don't create mixer volume elements for Headphone and Speaker if they
use the same DAC as normal line-outs on AD1988.  Otherwise the amp
value gets screwed up, e.g.
	https://bugzilla.novell.com/show_bug.cgi?id=398255

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>