acpi_spmc(4): Fix comments on constraints storage

The fields themselves are not unused, we actually fill them, but once
filled we indeed do not use their values (yet). See the '#ifdef notyet'
section in acpi_spmc_check_constraints().

No functional change.

Reviewed by: emaste, obiwac, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56798