libefivar: Fix incorrect check for DisplayOnly text format in AcpiEx

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.

Upstream Bug: https://bugzilla.tianocore.org/show_bug.cgi?id=1312
Obtained from: https://github.com/tianocore/edk2/commit/e9ab1635a2cfa1cb588996dcd5cc157fb44decb1
Pull Request: https://github.com/freebsd/freebsd-src/pull/581