ALSA: firewire-lib: Split some codes into functions to reuse for both streams

Some codes can be reused to handle in-stream. This commit adds new functions.
This commit also renames some functions to keep naming consistency.

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