ALSA: firewire-digi00x: code refactoring to finish streaming session

This commit is a part of preparation to perform allocation/release
of isochronous resources in pcm.hw_params/hw_free callbacks.

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>