usb: dwc3: gadget: Store resource index of start cmd

As long as the START_TRANSFER command completes, it provides the
resource index of the endpoint. Use this when we need to issue
END_TRANSFER command to an isoc endpoint to retry with a new
XferNotReady event.

Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>