speaker-test: Show out-of-chmap channels as Unknown

speaker-test: Show out-of-chmap channels as Unknown

Currently speaker-test falls back to ALSA default channel names for
channels out-of-chmap.

This causes e.g. the 4th channel of
$ speaker-test -c4 -Dhdmi -m "FR,FL,FC"
to be shown as "Rear Right".

Change the code to show such channels as Unknown instead, similar to
when snd_pcm_chmap_long_name() does not know the name.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>