gpio: mark INVARIANTS variables as __diagused

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

Reviewed by: dim
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36097

(cherry picked from commit 7dc4d5118c0268efe48c0e36c49cba1e2a8e39fc)