ALSA: firewire-tascam: minor code refactoring to finish streaming session

The operation to finish packet streaming corresponds to stopping
isochronous contexts.

This commit applies code refactoring to move codes to stop into
a helper function to finish the session.

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