mptable_hostb: Use legacy_get_pcibus() to fetch PCI bus number.

The mptable_hostb driver is a child of legacy0 and has legacy bus
ivars, not PCI or PCI bridge ivars.

PR: 264819
Reported by: Dennis Clarke <dclarke@blastwave.org>
Diagnosed by: avg
Reviewed by: avg
Differential Revision: https://reviews.freebsd.org/D35548

(cherry picked from commit b076d8d54c2fd91a35aad6931d0bfa2aa0122036)