ao_alsa: treat SND_CHMAP_UNKNOWN as NA channel too

Apparently required by nVidia HDMI. It should not be, and NA would
definitely be more correct here, so this could be considered a driver
bug. Maybe.

audio/out/ao_alsa.c

@@ -275,6 +275,7 @@ static const int alsa_to_mp_channels[][2] = {
     {SND_CHMAP_RLC,     MP_SP(SDL)},
     {SND_CHMAP_MONO,    MP_SP(FC)},
     {SND_CHMAP_NA,      MP_SPEAKER_ID_NA},
+    {SND_CHMAP_UNKNOWN, MP_SPEAKER_ID_NA},
     {SND_CHMAP_LAST,    MP_SPEAKER_ID_COUNT}
 };