libefivar: Change OPTIONAL keyword usage style
Update all use of ', OPTIONAL' to ' OPTIONAL,' for function params.
Upstream Bug: https://bugzilla.tianocore.org/show_bug.cgi?id=3760
Obtained from: https://github.com/tianocore/edk2/commit/d0e2f8232a26453fc0191629ed44ff2a46ea073e
Pull Request: https://github.com/freebsd/freebsd-src/pull/581