MdePkg: Fix incorrect check for DisplayOnly text format in AcpiEx

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1312

Text format for AcpiEx device path in UEFI Spec:
AcpiEx(HID,CID,UID,HIDSTR,CIDSTR,UIDSTR)
AcpiEx(HID|HIDSTR,(CID|CIDSTR,UID|UIDSTR))(Display Only)

When convert device path to text for AcpiEx device path,
current code check AllowShortcuts parameter to convert
the device path to DisplayOnly text format(shorter text
representation) by mistake.
It should check DisplayOnly parameter.

This commit is to fix this issue.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/UefiDevicePathLib/DevicePathToText.c

@@ -497,7 +497,7 @@ DevPathToTextAcpiEx (
        );
     }
   } else {
-    if (AllowShortcuts) {
+    if (DisplayOnly) {
       //
       // display only
       //