thunderbolt: Fix INVARIANTS compilation

The problem is that THUNDERBOLT_DEBUG triggers the use of some
variables, but it is independent of INVARIANTS and the variables it uses
were tagged with '__diagused'.

Fix this by using '__maybe_unused' in those places.

Fixes: 183633079178 ("thunderbolt: make code -Wunused clean")
Fixes: 886164895f3f ("amd64: complete thunderbolt KERNCONF integration")
Sponsored by: The FreeBSD Foundation