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

sys/x86/include/apicvar.h

@@ -78,7 +78,12 @@
 #define	MAX_APIC_ID		0x800
 #define	APIC_ID_ALL		0xffffffff
 
-#define	IOAPIC_MAX_ID		xAPIC_MAX_APIC_ID
+/*
+ * The 0xff ID is used for broadcast IPIs for local APICs when not using
+ * x2APIC.  IPIs are not sent to I/O APICs so it's acceptable for an I/O APIC
+ * to use that ID.
+ */
+#define	IOAPIC_MAX_ID		0xff
 
 /* I/O Interrupts are used for external devices such as ISA, PCI, etc. */
 #define	APIC_IO_INTS	(IDT_IO_INTS + 16)