[3006.x] Allow for secure-boot efivars directory having SecureBoot-xxx files, not directories with a data file

[dmurphy18]

What does this PR do?

Correctly reads SecureBoot files from an efivars directory, previously attempted to treat them as directories with data files, that is what is in the old secure boot vars directory

What issues does this PR fix or reference?

Fixes #66955

Previous Behavior

Would report efi-secure-boot grain as False, when it should have reported True

New Behavior

Now reports efi-secure-boot grain as True, when it is True

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

Please review Salt's Contributing Guide for best practices, including the
PR Guidelines.

See GitHub's page on GPG signing for more information about signing commits with GPG.