acpi_spmc(4): Remove XXX from a comment about retrieving constraints

There is really nothing we can do about a DSM function that works on
first call but fails on subsequent calls, except calling it only once,
which we already do.

While here, soften the comment message, as failure was observed with
some specific machines only.

No functional change.

Reviewed by: emaste, obiwac
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56799