Drivers/DwUsb3Dxe: fix uninitialized variable

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
diff --git a/Drivers/Usb/DwUsb3Dxe/DwUsb3Dxe.c b/Drivers/Usb/DwUsb3Dxe/DwUsb3Dxe.c
index 35d61d6..7695112 100644
--- a/Drivers/Usb/DwUsb3Dxe/DwUsb3Dxe.c
+++ b/Drivers/Usb/DwUsb3Dxe/DwUsb3Dxe.c
@@ -778,6 +778,7 @@
     break;
   default:
     DEBUG ((DEBUG_ERROR, "DwUsb3GetDeviceSpeed: invalid DSTS:0x%x\n", Data));
+    Speed = USB_SPEED_UNKNOWN;
     break;
   }
   return Speed;