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