firewire-motu: fix wrong reference count for stream functionality at error...

firewire-motu: fix wrong reference count for stream functionality at error path of rawmidi interface

In IEC 61883-6, several types of sampling data can be multiplexed into
payload of common isochronous packet (CIP). For typical audio and music
units, PCM samples and MIDI messages are multiplexed into one packet
streaming.

ALSA firewire-motu driver allows applications of rawmidi interface to
start packet streaming for transmission of MIDI messages. However at
error path, the reference count of stream functionality is not operated
correctly. This can brings a bug that packet streaming is not stopped
when all referrers release the count.

This commit fixes the bug.

Fixes: 8edc56ec ("ALSA: firewire-motu: reserve/release isochronous resources in pcm.hw_params/hw_free callbacks")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/firewire/motu/motu-midi.c

@@ -22,6 +22,8 @@ static int midi_open(struct snd_rawmidi_substream *substream)
 	if (err >= 0) {
 		++motu->substreams_counter;
 		err = snd_motu_stream_start_duplex(motu);
+		if (err < 0)
+			--motu->substreams_counter;
 	}
 
 	mutex_unlock(&motu->mutex);