hw/audio/hda-codec: reenable the audio mixer

hw/audio/hda-codec: reenable the audio mixer

Commit b7639b7dd0 ("hw/audio: Simplify hda audio init") inverted
the sense of hda codec property mixer during initialization.
Change the code so that mixer=on enables the hda mixer emulation
and mixer=off disables the hda mixer emulation.

With this change audio playback and recording streams don't start
muted by default.

Fixes: b7639b7dd0 ("hw/audio: Simplify hda audio init")
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20231105172552.8405-2-vr_qemu@t-online.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>