usb: dwc3: gadget: when the started list is empty stop the active xfer

When we have nothing left to be queued after handling the last trb
we have to stop the current transfer. This way we can ensure that
the next request will be queued with a new and valid timestamp
and will not directly run into an missed xfer.

Reviewed-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@kernel.org>