usb: gadget: udc: core: update usb_ep_queue() documentation

Mention that ->complete() should never be called from within
usb_ep_queue().

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>