MdeModulePkg/UsbKb: ReadKeyStrokeEx always return key state

Today's implementation only return key state when there is key.
But when user doesn't press any key, the key state cannot be
returned.

The patch changes the ReadKeyStrokeEx() to always return the
key state even there is no key pressed.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>

MdeModulePkg/Bus/Usb/UsbKbDxe/KeyBoard.h

@@ -1,7 +1,7 @@
 /** @file
   Function prototype for USB Keyboard Driver.
 
-Copyright (c) 2004 - 2013, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2004 - 2018, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
 which accompanies this distribution.  The full text of the license may be found at
@@ -317,4 +317,16 @@ SetKeyLED (
   IN  USB_KB_DEV    *UsbKeyboardDevice
   );
 
+/**
+  Initialize the key state.
+
+  @param  UsbKeyboardDevice     The USB_KB_DEV instance.
+  @param  KeyState              A pointer to receive the key state information.
+**/
+VOID
+InitializeKeyState (
+  IN  USB_KB_DEV           *UsbKeyboardDevice,
+  OUT EFI_KEY_STATE        *KeyState
+  );
+
 #endif