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>

arch/x86/kernel/apic/apic.c

@@ -2249,7 +2249,7 @@ void __init apic_set_eoi_write(void (*eoi_write)(u32 reg, u32 v))
 static void __init apic_bsp_up_setup(void)
 {
 #ifdef CONFIG_X86_64
-	apic_write(APIC_ID, SET_APIC_ID(boot_cpu_physical_apicid));
+	apic_write(APIC_ID, apic->set_apic_id(boot_cpu_physical_apicid));
 #else
 	/*
 	 * Hack: In case of kdump, after a crash, kernel might be booting