ALSA: firewire-tascam: apply small refactoring to handle several type of event for hwdep interface

In later commits, ALSA firewire-tascam driver will allow userspace
applications to receive notifications about changes of device state,
transferred in tx isochronous packet. At present, all of drivers in ALSA
firewire stack have mechanism to notify change of status of packet
streaming, thus it needs to distinguish these two types of notification.

This commit is a preparation for the above.

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