usb: fix a typo in usb_class_driver documentation

replace usb_unregister_dev by usb_deregister_dev
Signed-off-by: Amitesh Singh <singh.amitesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb.h

@@ -1160,7 +1160,7 @@ extern struct bus_type usb_bus_type;
  * @minor_base: the start of the minor range for this driver.
  *
  * This structure is used for the usb_register_dev() and
- * usb_unregister_dev() functions, to consolidate a number of the
+ * usb_deregister_dev() functions, to consolidate a number of the
  * parameters used for them.
  */
 struct usb_class_driver {