efi: Update stale locking comment for struct efivars

The comment describing how struct efivars->lock is used hasn't been
updated in sync with the code. Fix it.

Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Mike Waychison <mikew@google.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>