x86: don't call read_apic_id if !cpu_has_apic

should not call that if apic is disabled.

[ Impact: fix crash on certain UP configs ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
LKML-Reference: <4A09CCBB.2000306@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>