hda-audio: fix non-mixer codecs

They don't advertise mixer support, but still allow the guest change
mixer settings.  Add a check to avoid it.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Merge remote-tracking branch 'remotes/kraxel/tags/pull-audio-4' into staging

hda-audio: fix non-mixer codecs

# gpg: Signature made Tue 29 Apr 2014 10:03:44 BST using RSA key ID D3E87138
# gpg: Good signature from "Gerd Hoffmann (work) <kraxel@redhat.com>"
# gpg:                 aka "Gerd Hoffmann <gerd@kraxel.org>"
# gpg:                 aka "Gerd Hoffmann (private) <kraxel@gmail.com>"

* remotes/kraxel/tags/pull-audio-4:
  hda-audio: fix non-mixer codecs

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>