ALSA: firewire-lib: Fix 'amdtp_domain_start()' when no AMDTP_OUT_STREAM stream is found

The intent here is to return an error code if we don't find what we are
looking for in the 'list_for_each_entry()' loop.

's' is not NULL if the list is empty or if we scan the complete list.
Introduce a new 'found' variable to handle such cases.

Fixes: 60dd4929 ("ALSA: firewire-lib: handle several AMDTP streams in callback handler of IRQ target")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/9c9a53a4905984a570ba5672cbab84f2027dedc1.1624560484.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Takashi Iwai <tiwai@suse.de>