x86/apic: Fix typo EIO_ACK -> EOI_ACK and document it

Fix typo in the macro name and document the
reason it has this value. Update users.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: gleb@redhat.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/37867b31b9330690af2e60a2a7c4cb4b1b070caf.1337184153.git.mst@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/apicdef.h b/arch/x86/include/asm/apicdef.h
index 134bba0..c46bb99 100644
--- a/arch/x86/include/asm/apicdef.h
+++ b/arch/x86/include/asm/apicdef.h
@@ -37,7 +37,7 @@
 #define		APIC_ARBPRI_MASK	0xFFu
 #define	APIC_PROCPRI	0xA0
 #define	APIC_EOI	0xB0
-#define		APIC_EIO_ACK		0x0
+#define		APIC_EOI_ACK		0x0 /* Docs say 0 for future compat. */
 #define	APIC_RRR	0xC0
 #define	APIC_LDR	0xD0
 #define		APIC_LDR_MASK		(0xFFu << 24)