USB: gadget: core: Add missing kerneldoc for vbus_work

Add a missing kerneldoc description of the vbus_work field in struct usb_udc.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Fixes: 50966da8 ("usb: gadget: udc: core: Offload usb_udc_vbus_handler processing")
Link: https://lore.kernel.org/r/1e5e7cda-b2c8-4917-9952-4354f365ede0@rowland.harvard.eduSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/core.c

@@ -40,6 +40,7 @@ static const struct bus_type gadget_bus_type;
  * @allow_connect: Indicates whether UDC is allowed to be pulled up.
  * Set/cleared by gadget_(un)bind_driver() after gadget driver is bound or
  * unbound.
+ * @vbus_work: work routine to handle VBUS status change notifications.
  * @connect_lock: protects udc->started, gadget->connect,
  * gadget->allow_connect and gadget->deactivate. The routines
  * usb_gadget_connect_locked(), usb_gadget_disconnect_locked(),