ALSA: bebob: Use different labels for digital input/output

This commit uses different labels for control elements of digital input/output
interfaces to correct my misunderstanding about M-Audio Firewire 1814 and
ProjectMix I/O.

According to user manuals for these two models, they have two modes for
digital input; one is S/PDIF in both of optical and coaxial interfaces,
another is ADAT in optical interface only.

But in current implementation, a control element for it reduced labels which
a control element for digital output uses because of my misunderstanding
that optical interface is not available for digital input with S/PDIF mode.

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