Increase IOAPIC_MAX_ID to 255 (from 254)

A test system provided by AMD panicked with "madt_parse_apics:
I/O APIC ID 255 too high". I/O APIC ID 255 is acceptable, so increase
the limit.

Reviewed by: jhb, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45157