USB: core: fix sparse warning for static function

Fix the following sparse warning:

drivers/usb/core/usb.c:1033:15: warning: symbol 'usb_debug_devices' was not declared. Should it be static?
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/usb.c

@@ -1067,7 +1067,7 @@ static struct notifier_block usb_bus_nb = {
 struct dentry *usb_debug_root;
 EXPORT_SYMBOL_GPL(usb_debug_root);
 
-struct dentry *usb_debug_devices;
+static struct dentry *usb_debug_devices;
 
 static int usb_debugfs_init(void)
 {