libefivar: Correct condition check for AcpiExp text format

According to UEFI Spec, for ACPI Expanded Device Path,
when HIDSTR=empty, CIDSTR=empty, UID STR!=empty,
the ACPI Expanded Device Path node can be displayed as
AcpiExp(HID,CID,UIDSTR) format.
And if UID is 0 and UIDSTR is empty, then use AcpiEx format.

This patch is to correct the condition check to follow UEFI
Spec when convert the device path node to the AcpiExp text
format.

Upstream Bug:	https://bugzilla.tianocore.org/show_bug.cgi?id=1226
Obtained from:	fb4bea551e
Pull Request:   https://github.com/freebsd/freebsd-src/pull/581

lib/libefivar/efivar-dp-format.c

@@ -514,7 +514,7 @@ DevPathToTextAcpiEx (
     (AcpiEx->CID >> 16) & 0xFFFF
     );
 
-  if ((*HIDStr == '\0') && (*CIDStr == '\0') && (AcpiEx->UID == 0)) {
+  if ((*HIDStr == '\0') && (*CIDStr == '\0') && (*UIDStr != '\0')) {
     //
     // use AcpiExp()
     //