ALSA: usb-audio: Avoid unnecessary interface change at EP close

We toggle USB interface at PCM prepare and reset at close.  When the
PCM isn't prepared, resetting again makes little sense.
Check the current altset and avoid unnecessary interface reset at EP
close.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

Author: tiwai

sound/usb/endpoint.c

@@ -32,6 +32,7 @@ struct snd_usb_iface_ref {
 	unsigned char iface;
 	bool need_setup;
 	int opened;
+	int altset;
 	struct list_head list;
 };
 
@@ -899,6 +900,9 @@ static int endpoint_set_interface(struct snd_usb_audio *chip,
 	int altset = set ? ep->altsetting : 0;
 	int err;
 
+	if (ep->iface_ref->altset == altset)
+		return 0;
+
 	usb_audio_dbg(chip, "Setting usb interface %d:%d for EP 0x%x\n",
 		      ep->iface, altset, ep->ep_num);
 	err = usb_set_interface(chip->dev, ep->iface, altset);
@@ -910,6 +914,7 @@ static int endpoint_set_interface(struct snd_usb_audio *chip,
 
 	if (chip->quirk_flags & QUIRK_FLAG_IFACE_DELAY)
 		msleep(50);
+	ep->iface_ref->altset = altset;
 	return 0;
 }