ACPI: processor: refactor internal map_x2apic_id()

Untangle the nested if conditions to make this function look
more similar to the other map_*apic_id() functions.

Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>