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=398255Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@perex.cz>