xen/cpufreq: neglect unsupported-mode request from DOM0

DOM0 could deliever whatever performance statistic (Px, _CPC) it parses, it is
Xen's responsibility to decide which one it shall accept.
Xen rely on XEN_PROCESSOR_PM_xxx flag to tell which mode ( Px or CPPC )
current cpufreq driver supports, and accepts relative info. It will neglect
unsupported-mode request and yields success.

Signed-off-by: Penny Zheng <Penny.Zheng@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/platform_hypercall.c

@@ -539,9 +539,14 @@ ret_t do_platform_op(
         case XEN_PM_PX:
             if ( !(xen_processor_pmbits & XEN_PROCESSOR_PM_PX) )
             {
-                ret = -ENOSYS;
+                /*
+                 * Neglect Px-info when registered cpufreq driver
+                 * isn't in Px mode
+                 */
+                ret = 0;
                 break;
             }
+
             ret = set_px_pminfo(op->u.set_pminfo.id, &op->u.set_pminfo.u.perf);
             break;