x86/amd: Fix inverted condition in zenbleed_use_chickenbit()

The identified commit inverted the condition under which Xen chooses
to set the Zenbleed chickenbit.

Undo that, and leave a better comment explaining why.

Found by AI review of the referenced commit.

This is XSA-515 / CVE-2026-79608.

Fixes: d0c75dc4c0 ("x86/amd: Fix race editing DE_CFG")
Assisted-by: Sashiko + Opus 5.0
Reported-by: George Dunlap <dunlapg@umich.edu>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 0c5ec39d91)

xen/arch/x86/cpu/amd.c

@@ -992,7 +992,12 @@ static bool zenbleed_use_chickenbit(void)
         return is_zen2_uarch();
     }
 
-    return (uint8_t)curr_rev >= fixed_rev;
+    /*
+     * Zenbleed can be mitigated by disabling Floating-Point
+     * Mov-Elimination, but this is expensive.  Only do so when we
+     * don't have a fix in microcode.
+     */
+    return (uint8_t)curr_rev < fixed_rev;
 }
 
 void amd_init_de_cfg(const struct cpuinfo_x86 *c)