libefivar: Correct the string expression of UTF8 vendor device path

According to UEFI spec, the string expression of UTF8 vendor
device node should be displayed as: VenUtf8(). Current code
display it as: VenUft8() by mistake when convert device
path node to text.

This commit is to fix this bug.

Upstream Bug: https://bugzilla.tianocore.org/show_bug.cgi?id=1225
Obtained from: https://github.com/tianocore/edk2/commit/959be180e185869b71db9dad34c3f4bba660d724
Pull Request: https://github.com/freebsd/freebsd-src/pull/580

(cherry picked from commit 91a35e5803bef59b8ca67b0621216124cc2e42f8)