usb: dwc3: gadget: don't issue End Transfer if we have started reqs

In case we have many started requests and one of them in the middle is
completed with Missed Isoc, let's not End Transfer as that would
result in us loosing (possibly) many more intervals.

Instead, let's allow the controller to go through its list of started
requests.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>