usb: dwc3: gadget: req->queued must be forced to false in cleanup

I am not sure, why I found it during SG debugging. But, I noticed that
even when req_queued list was empty, there were some request in
request_list having queued flag true. If I run test second time, it
first removes all request from request_list and hence busy_slot was
wrongly incremented.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>