ALSA: firewire-lib: operate data block counter in top level of processing for IT context

In ALSA IEC 61883-1/6 packet streaming engine, two types of data block
counter are supported. This commit applies code refactoring to make it
explicitly for IT context.

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