From: James Cleverdon <jamesclv@us.ibm.com>

32-way IBM x445s will have I/O xAPICs with IDs greater than 0xF (0x8 to 0xE
isn't enough).  This breaks the code in setup_ioapic_ids_from_mpc.
However, the entire unique ID check is unnecessary.  Only I/O APICs using
the serial APIC bus need the unique numbers.  Those sending messages
through the system bus simply don't use them.