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

sys/x86/x86/mptable.c

@@ -1250,7 +1250,7 @@ mptable_pci_host_res_init(device_t pcib)
 	struct host_res_args args;
 
 	KASSERT(pci0 != -1, ("do not know how to map PCI bus IDs"));
-	args.bus = pci_get_bus(pcib) + pci0;
+	args.bus = legacy_get_pcibus(pcib) + pci0;
 	args.dev = pcib;
 	args.sc = device_get_softc(pcib);
 	if (pcib_host_res_init(pcib, &args.sc->sc_host_res) != 0)

sys/x86/x86/mptable_pci.c

@@ -57,7 +57,7 @@ mptable_hostb_probe(device_t dev)
 
 	if (pci_cfgregopen() == 0)
 		return (ENXIO);
-	if (mptable_pci_probe_table(pcib_get_bus(dev)) != 0)
+	if (mptable_pci_probe_table(legacy_get_pcibus(dev)) != 0)
 		return (ENXIO);
 	device_set_desc(dev, "MPTable Host-PCI bridge");
 	return (0);