acpi: Don't use MAX_CPUMASK_BITS for APIC ID bitmap

MAX_CPUMASK_BITS is a limit for max_cpus and CPU indexes, not for APIC
IDs.

ACPI_CPU_HOTPLUG_ID_LIMIT is the right macro for the limit on APIC IDs
on the ACPI and CPU hotplug code.

There are no functional changes introduced by this patch, as
MAX_CPUMASK_BITS + 1 == 255 + 1 == 256 == ACPI_CPU_HOTPLUG_ID_LIMIT.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
