usb: dwc3: gadget: Fix double add due to cleanup_cancelled_request

Commit 6a247993218c ("usb: dwc3: gadget: Properly handle failed
kick_transfer") introduces a piece of code where if gadget_ep_cmd
fails in kick_transfer, it calls stop_active_transfer for that ep
& move the request from started_list to cancelled_list. Finally
it checks for DWC3_EP_END_TRANSFER_PENDING flag, if not set then
moves ahead and calls dwc3_gadget_cleanup_cancelled_request as part
of which gadget giveback for the request is called. This will
ultimately mean ep_queue failure as ep_queue returns the status of
kick_transfer.

Now from the function driver perspective, when ep_queue fails
it tries to add the request back to the pool from which the request
was queued.

But,since we did a cleanup_cancelled_request the completion handler
of the function driver is also called which tries to add the same
request back to the pool, resulting in a double add scenario.

Fix this by returning 0 from ep_queue rather then returning the
status of kick_transfer. For a function driver it would mean
ep_queue success and will not add the request back to read pool
and since we already are giving back the request as part of
cleanup_cancelled_requests, the request will be added back to
the pool and avoid the double add scenario.

Change-Id: I82bc21c34a795820998483c4a88e1891495a6906
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>