x86/HWP: adjust feature_hdc's section annotation

The variable can be cleared (set to false) by a non-init function's error
path (hwp_init_msrs()).

Fixes: 99c4570f82 ("cpufreq: Add Hardware P-State (HWP) driver")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Release-Acked-by: Oleksii Kurochko<oleksii.kurochko@gmail.com>

xen/arch/x86/acpi/cpufreq/hwp.c

@@ -21,7 +21,7 @@ static bool __ro_after_init hwp_in_use;
 static bool __ro_after_init feature_hwp_notification;
 static bool __ro_after_init feature_hwp_activity_window;
 
-static bool __ro_after_init feature_hdc;
+static bool __read_mostly feature_hdc;
 
 static bool __ro_after_init opt_cpufreq_hdc = true;