Impact: cleanup

this warning:

  arch/x86/kernel/early-quirks.c:99: warning: ‘ati_ixp4x0_rev’ defined but not used

triggers because ati_ixp4x0_rev() is only used in the
ACPI && X86_IO_APIC case.
Signed-off-by: NIngo Molnar <mingo@elte.hu>