usb: dwc3: ep0: prevent starting transfers twice on ep0

In case we try to start an invalid test mode, we
will call dwc3_ep0_stall_and_restart() but we will
also call dwc3_ep0_out_start() which will start
a second transfer on ep0.

Let's prevent any problems by returning early in
the error case.
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/ep0.c

@@ -744,6 +744,7 @@ static void dwc3_ep0_complete_status(struct dwc3 *dwc,
 			dev_dbg(dwc->dev, "Invalid Test #%d\n",
 					dwc->test_mode_nr);
 			dwc3_ep0_stall_and_restart(dwc);
+			return;
 		}
 	}