[Bluetooth] Fix disconnect race on ISOC interface

The hci_usb_disconnect() gets called recursively when SCO support
is enabled and used. This causes sysfs_hash_and_remove() finally to
dereference a NULL pointer.

Noticed by Sebastian Schmidt <yath@yath.eu.org>

drivers/bluetooth/hci_usb.c

@@ -976,11 +976,13 @@ int hci_usb_probe(struct usb_interface *intf, const struct usb_device_id *id)
 static void hci_usb_disconnect(struct usb_interface *intf)
 {
 	struct hci_usb *husb = usb_get_intfdata(intf);
-	struct hci_dev *hdev = husb->hdev;
+	struct hci_dev *hdev;
 
-	if (!husb)
+	if (!husb || intf == husb->isoc_iface)
 		return;
+
 	usb_set_intfdata(intf, NULL);
+	hdev = husb->hdev;
 
 	BT_DBG("%s", hdev->name);