ALSA: usb: Only get AudioControl header for UAC1 class.

The control header needs to be read from buffer at this point only
in the case of UAC1 protocol. Move it inside the switch case as other
protocols such as the Basic Audio Device spec will have an empty buffer
that is latter filled as inferred.
Signed-off-by: Jorge Sanjuan <jorge.sanjuan@codethink.co.uk>
[Ruslan: updated with recently added sanity checks]
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/card.c

@@ -221,25 +221,34 @@ static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
 	struct usb_device *dev = chip->dev;
 	struct usb_host_interface *host_iface;
 	struct usb_interface_descriptor *altsd;
-	void *control_header;
 	int i, protocol;
-	int rest_bytes;
 
 	/* find audiocontrol interface */
 	host_iface = &usb_ifnum_to_if(dev, ctrlif)->altsetting[0];
-	control_header = snd_usb_find_csint_desc(host_iface->extra,
-						 host_iface->extralen,
-						 NULL, UAC_HEADER);
 	altsd = get_iface_desc(host_iface);
 	protocol = altsd->bInterfaceProtocol;
 
-	if (!control_header) {
+	switch (protocol) {
+	default:
+		dev_warn(&dev->dev,
+			 "unknown interface protocol %#02x, assuming v1\n",
+			 protocol);
+		/* fall through */
+
+	case UAC_VERSION_1: {
+		struct uac1_ac_header_descriptor *h1;
+		int rest_bytes;
+
+		h1 = snd_usb_find_csint_desc(host_iface->extra,
+							 host_iface->extralen,
+							 NULL, UAC_HEADER);
+		if (!h1) {
 			dev_err(&dev->dev, "cannot find UAC_HEADER\n");
 			return -EINVAL;
 		}
 
-	rest_bytes = (void *)(host_iface->extra + host_iface->extralen) -
-		control_header;
+		rest_bytes = (void *)(host_iface->extra +
+				host_iface->extralen) - (void *)h1;
 
 		/* just to be sure -- this shouldn't hit at all */
 		if (rest_bytes <= 0) {
@@ -247,16 +256,6 @@ static int snd_usb_create_streams(struct snd_usb_audio *chip, int ctrlif)
 			return -EINVAL;
 		}
 
-	switch (protocol) {
-	default:
-		dev_warn(&dev->dev,
-			 "unknown interface protocol %#02x, assuming v1\n",
-			 protocol);
-		/* fall through */
-
-	case UAC_VERSION_1: {
-		struct uac1_ac_header_descriptor *h1 = control_header;
-
 		if (rest_bytes < sizeof(*h1)) {
 			dev_err(&dev->dev, "too short v1 buffer descriptor\n");
 			return -EINVAL;