x86, ioapic: Don't warn about non-existing IOAPICs if we have none

mp_find_ioapic() prints errors like:

    ERROR: Unable to locate IOAPIC for GSI 13

if it can't find the IOAPIC that manages that specific GSI. I
see errors like that at every boot of a laptop that apparently
doesn't have any IOAPICs.

But if there are no IOAPICs it doesn't seem to be an error that
none can be found. A solution that gets rid of this message is
to directly return if nr_ioapics (still) is zero. (But keep
returning -1 in that case, so nothing breaks from this change.)

The call chain that generates this error is:

pnpacpi_allocated_resource()
    case ACPI_RESOURCE_TYPE_IRQ:
        pnpacpi_parse_allocated_irqresource()
            acpi_get_override_irq()
                 mp_find_ioapic()

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>