ALSA: bebob: fix wrong decoding of clock information for Terratec PHASE 88 Rack FW

Terratec PHASE 88 rack fw has two registers for source of clock, one is
for internal/external, and another is for wordclock/spdif for external.

When clock source is internal, information in another register has no meaning.
Thus it must be ignored, but current implementation decodes it. This causes
over-indexing reference to labels.

Reported-by: András Murányi <muranyia@gmail.com>
Tested-by: András Murányi <muranyia@gmail.com>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>