usb: gadget: udc: core: Document the relation between usb_ep_queue() and completion callback

Improve the kerneldoc for usb_ep_queue() to note explicitly that the
request's completion routine will be called if and only if the return
value is 0.  The corresponding fact about usb_submit_urb() for the
host-side API has long been documented, and this has always been the
intention for the gadget API.  But until now, documentation seems to
have been lacking.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>