ALSA: usb6fire: prevent driver panic state when stopping

The patch below prevents the 6fire usb driver going into panic state
when stopping playing. On some systems the urb in handler
(usb6fire_pcm_in_urb_handler) is being called while urbs are being
killed off, this causes the driver to set panic state and can result in
the kernel warning 'URB %p submitted while active'.
Signed-off-by: NJurgen Kramer <gtmkramer@xs4all.nl>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/usb/6fire/pcm.c

@@ -135,6 +135,9 @@ static void usb6fire_pcm_stream_stop(struct pcm_runtime *rt)
 	struct control_runtime *ctrl_rt = rt->chip->control;
 
 	if (rt->stream_state != STREAM_DISABLED) {
+
+		rt->stream_state = STREAM_STOPPING;
+
 		for (i = 0; i < PCM_N_URBS; i++) {
 			usb_kill_urb(&rt->in_urbs[i].instance);
 			usb_kill_urb(&rt->out_urbs[i].instance);