x86: numachip: Fix 16-bit APIC ID truncation

Prevent 16-bit APIC IDs being truncated by using correct mask. This fixes
booting large systems, where the wrong core would receive the startup and
init IPIs, causing hanging.

Signed-off-by: Daniel J Blueman <daniel@numascale.com>
Cc: Steffen Persvold <sp@numascale.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/1415089784-28779-1-git-send-email-daniel@numascale.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>