x86/apic: Remove the SET_APIC_ID(x) macro

The SET_APIC_ID() macro obfusates the code. Remove it to increase
readability and add a comment to the apic struct to document that the
callback is required on 64-bit.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Link: http://lkml.kernel.org/r/1488971270-14359-1-git-send-email-douly.fnst@cn.fujitsu.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>