Ensure that mds_handler always points to a valid method.

Depending on system configuration, version, and architecture,
mds_handler might be dereferenced from doreti before
hw_mds_recalculate_boot() initialized it. Statically assign void
method to cover all cases.

Reported by: "Schuendehuette, Matthias (LDA IT PLM)" <matthias.schuendehuette@siemens.com>
Sponsored by: The FreeBSD Foundation
MFC after: 3 days