ALSA: bebob: apply new enumerator to normalize the type of clock source

Previous commit allows this driver to detect several types of clock
source, while there's no normalized expression for it.

This commit adds a new enumerator for this purpose.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>