x86: fix sparse warning in xen/time.c

Use xen_khz to denote xen_specific clock speed.  Avoid shadowing
cpu_khz.

arch/x86/xen/time.c:220:6: warning: symbol 'cpu_khz' shadows an earlier one
include/asm/tsc.h:17:21: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/xen/time.c

@@ -217,17 +217,17 @@ unsigned long long xen_sched_clock(void)
 /* Get the CPU speed from Xen */
 unsigned long xen_cpu_khz(void)
 {
-	u64 cpu_khz = 1000000ULL << 32;
+	u64 xen_khz = 1000000ULL << 32;
 	const struct vcpu_time_info *info =
 		&HYPERVISOR_shared_info->vcpu_info[0].time;
 
-	do_div(cpu_khz, info->tsc_to_system_mul);
+	do_div(xen_khz, info->tsc_to_system_mul);
 	if (info->tsc_shift < 0)
-		cpu_khz <<= -info->tsc_shift;
+		xen_khz <<= -info->tsc_shift;
 	else
-		cpu_khz >>= info->tsc_shift;
+		xen_khz >>= info->tsc_shift;
 
-	return cpu_khz;
+	return xen_khz;
 }
 
 /*