usb: gadget: composite: dequeue cdev->req before free its buffer

usb: gadget: composite: dequeue cdev->req before free its buffer

commit c6876fd(usb: gadget: composite: dequeue cdev->req before free it in
composite_dev_cleanup) fixed a bug: free the usb request(i.e. cdev->req) but
does not dequeue it beforehand. This fix is not proper enough because it
dequeues the request after free its data buffer, considering the hardware can
access the buffer's memory anytime before the request's complettion rountine
runs, and usb_ep_dequeue always call the complettion rountine before it returns,
so the best way is to dequeue the request before free its buffer.

Suggested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Li Jun <b47624@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>