usb: f_qdss: Handle async completion of requests during qdss_close

When qdss_close is called we try to dequeue the pending and started
requests and then free those requests. But when async completion is
enabled the completion callback could be called after freeing up
that request which may lead to list corruption. Handle this by
ensuring that the requests are freed only after the completion
callback is done for all the requests.

Change-Id: I3e5b1bad06f281422a71c84a617feb65027c2c84
Signed-off-by: Elson Roy Serrao <eserrao@codeaurora.org>