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.edu


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>