ALSA: dice: add 'firewire' directory for proc nodes

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>