ALSA: bebob/fireworks: simplify handling of local device entry table

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>