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

sound/firewire/amdtp-stream.c

@@ -1943,6 +1943,7 @@ int amdtp_domain_start(struct amdtp_domain *d, unsigned int tx_init_skip_cycles,
 	unsigned int events_per_period = d->events_per_period;
 	unsigned int queue_size;
 	struct amdtp_stream *s;
+	bool found = false;
 	int err;
 
 	if (replay_seq) {
@@ -1955,10 +1956,12 @@ int amdtp_domain_start(struct amdtp_domain *d, unsigned int tx_init_skip_cycles,
 
 	// Select an IT context as IRQ target.
 	list_for_each_entry(s, &d->streams, list) {
-		if (s->direction == AMDTP_OUT_STREAM)
+		if (s->direction == AMDTP_OUT_STREAM) {
+			found = true;
 			break;
+		}
 	}
-	if (!s)
+	if (!found)
 		return -ENXIO;
 	d->irq_target = s;