x86: Recover timer_ack lost in the merge of the NMI watchdog

In the course of the recent unification of the NMI watchdog an assignment
to timer_ack to switch off unnecesary POLL commands to the 8259A in the
case of a watchdog failure has been accidentally removed.  The statement
used to be limited to the 32-bit variation as since the rewrite of the
timer code it has been relevant for the 82489DX only.  This change brings
it back.
Signed-off-by: NMaciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/nmi.c

@@ -171,6 +171,9 @@ int __init check_nmi_watchdog(void)
 error:
 	if (nmi_watchdog == NMI_IO_APIC && !timer_through_8259)
 		disable_8259A_irq(0);
+#ifdef CONFIG_X86_32
+	timer_ack = 0;
+#endif
 	return -1;
 }