libefivar: Change OPTIONAL keyword usage style

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3760

Update all use of ', OPTIONAL' to ' OPTIONAL,' for function params.

Note that the link above no longer exists. The commit message was kept
verbatim. An archive of the bug report can be found at:
https://web.archive.org/web/20240714185609/https://bugzilla.tianocore.org/show_bug.cgi?id=3760

Obtained from: https://github.com/tianocore/edk2/commit/d0e2f8232a26453fc0191629ed44ff2a46ea073e

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1894