acpi: mark INVARIANTS variables as __diagused

Fixes INVARIANTS build with Clang 15, which previously failed due to
set-but-not-used variable warnings.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36096

(cherry picked from commit 0fd8d3589a68838d62d47f240d7fb22ef0d4dbeb)
(cherry picked from commit 9f45a241fc0db78ec62fd8ac3e19bc80694523f9)