x86/xen/time: cleanup xen_tsc_safe_clocksource

Modifies xen_tsc_safe_clocksource() to use newly defined constants from
arch/x86/include/asm/xen/cpuid.h.  This replaces a numeric value with
XEN_CPUID_TSC_MODE_NEVER_EMULATE, and deletes a comment that is now self
explanatory.

There should be no change in the function's behavior.
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/a69ca370fecf85d312d2db633d9438ace2af6e5b.1677038165.git.kjlx@templeofstupid.comSigned-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/time.c

@@ -20,6 +20,7 @@
 #include <asm/pvclock.h>
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
+#include <asm/xen/cpuid.h>
 
 #include <xen/events.h>
 #include <xen/features.h>
@@ -495,11 +496,7 @@ static int __init xen_tsc_safe_clocksource(void)
 	/* Leaf 4, sub-leaf 0 (0x40000x03) */
 	cpuid_count(xen_cpuid_base() + 3, 0, &eax, &ebx, &ecx, &edx);
 
-	/* tsc_mode = no_emulate (2) */
-	if (ebx != 2)
-		return 0;
-
-	return 1;
+	return ebx == XEN_CPUID_TSC_MODE_NEVER_EMULATE;
 }
 
 static void __init xen_time_init(void)