x86/apic: Use symbolic APIC_EOI_ACK

Use the symbol instead of hard-coded numbers,
now that the reason for the value is documented
where the constant is defined we don't need to
duplicate this explanation in code.

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/ecbe4c79d69c172378e47e5a587ff5cd10293c9f.1337184153.git.mst@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/apic.h

@@ -463,9 +463,7 @@ static inline void ack_APIC_irq(void)
 	 * ack_APIC_irq() actually gets compiled as a single instruction
 	 * ... yummie.
 	 */
-
-	/* Docs say use 0 for future compatibility */
-	apic_write(APIC_EOI, 0);
+	apic_write(APIC_EOI, APIC_EOI_ACK);
 }
 
 static inline unsigned default_get_apic_id(unsigned long x)