ALSA: bebob: Add static specifier to identifier with file scope

Some variables were declared without static even if they're not referred
to from external files. This commit make them local symbols for better
information-hiding by file unit.

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