ALSA: firewire-digi00x: unify stop and release method for duplex streams

>From callbacks for pcm and rawmidi interfaces, the functions to stop
and release duplex streams are called at the same time. This commit
merges the two functions.

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