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
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35548