In case we unload a gadget driver while any of
os_desc_req or req are still pending, we need
to make sure to dequeue them.

By using our setup_pending and os_desc_pending
flags we achieve that in a way that doesn't
cause any regressions because we won't dequeue
a request which was already completed.

The original idea came from Li Jun's commit
f2267089
(usb: gadget: composite: dequeue cdev->req
before free it in composite_dev_cleanup) which,
unfortunately, caused two regressions (kfree()
being called before usb_ep_dequeue() and calling
usb_ep_dequeue() when the request was already
completed). That commit also didn't take care
of os_desc_req which can fall into the same
situation so we must care for that one too.

Note that in order to make code slightly easier
to read, we introduce composite_ep_queue() which
hides details about how to fiddle with our pending
flags.
Signed-off-by: Felipe Balbi <balbi@ti.com>