ALSA: firewire-motu: print for unknown source of sampling clock

When unknown source is detected for sampling clock, corresponding label
was not added for node on procfs.

This commit adds it.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191030080644.1704-3-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>