ALSA: hda - Fix "PCM" name being used on one DAC when there are two DACs

In the scenario where there is one "Line Out", one "Speaker" and one
"Headphone", and there are only two DACs, two outputs will share a DAC.
Currently any mixer on such a DAC will get the "PCM" name, which is
misleading. Instead use "Headphone+LO" or "Speaker+LO" to better
specify what the volume actually controls.

[fixed missing slave string additions by tiwai]

Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>