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>
diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
index 9a8f26c..8244eb53 100644
--- a/drivers/usb/dwc3/ep0.c
+++ b/drivers/usb/dwc3/ep0.c
@@ -744,6 +744,7 @@
 			dev_dbg(dwc->dev, "Invalid Test #%d\n",
 					dwc->test_mode_nr);
 			dwc3_ep0_stall_and_restart(dwc);
+			return;
 		}
 	}