usb: dwc3: gadget: early giveback if End Transfer already completed

There is a rare race condition that may happen during a Disconnect
Interrupt if we have a started request that happens to be
dequeued *after* completion of End Transfer command. If that happens,
that request will be left waiting for completion of an End Transfer
command that will never happen.

If End Transfer command has already completed before, we are safe to
giveback the request straight away.

Tested-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>