In drivers of ALSA firewire stack, bebob and fireworks drivers have
local device entry table. At present, critical section to operate the
table is from the beginning/end of 'do_registration' call. This can be
more narrow and simplify codes.

This commit applies small refactoring for the above purpose.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>