xen: Fix misspelled CONFIG variable in comment.

Signed-off-by: NRobert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

arch/x86/xen/smp.c

@@ -361,7 +361,7 @@ static void xen_cpu_die(unsigned int cpu)
 		alternatives_smp_switch(0);
 }
 
-static void __cpuinit xen_play_dead(void) /* used only with CPU_HOTPLUG */
+static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU */
 {
 	play_dead_common();
 	HYPERVISOR_vcpu_op(VCPUOP_down, smp_processor_id(), NULL);