hpwmc: add __pmcdbg_used annotation

For variables which are only used in PMCDBG* macros.

Reviewed by: jkoshy, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40288

(cherry picked from commit 0589e7059bcd238027953d1311137512cc6dddea)