ACPI: disable ACPI cleanly when bad RSDP found

When ACPI is disabled in the BIOS of this VIA C3 box,
it invalidates the RSDP, which Linux notices:

ACPI Error (tbxfroot-0218): A valid RSDP was not found [20080926]

Bug Linux neglected to disable ACPI at that stage,
and later scribbled on smp_found_config:

ACPI: No APIC-table, disabling MPS

But this box doesn't run well in legacy PIC mode,
it needed IOAPIC mode to perform correctly:

http://lkml.org/lkml/2009/2/5/39

So exit ACPI mode cleanly when we first detect
that it is hopeless.

Signed-off-by: Len Brown <len.brown@intel.com>