ALSA: firewire: add const qualifier to identifiers for read-only symbols

Drivers in ALSA firewire stack still includes some symbols which can be
moved to a section for read-only symbols.

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