usb: dwc3: gadget: Don't clear flags before transfer ended

We track END_TRANSFER command completion. Don't clear transfer
started/ended flag prematurely. Otherwise, we'd run into the problem
with restarting transfer before END_TRANSFER command finishes.

Fixes: 6d8a0196 ("usb: dwc3: gadget: check for Missed Isoc from event status")
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>