Unlike the other drivers in ALSA firewire stack, ALSA dice driver does't
create 'firewire' directory for proc nodes because it has 'dice' node
only. But this is inconvenient because I have a plan to add another proc
node to output available stream formats from cache.

This commit let the driver to create the directory and put 'dice' node
into it.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>