x86/amd: Fix DE_CFG truncation in amd_check_zenbleed()

This line:

	val &= ~chickenbit;

ends up truncating val to 32 bits, and turning off various errata workarounds
in Zen2 systems.

Fixes: f91c5ea970 ("x86/amd: Mitigations for Zenbleed")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit c0dd53b8cb)

xen/arch/x86/cpu/amd.c

@@ -773,8 +773,8 @@ void __init detect_zen2_null_seg_behaviour(void)
 void amd_check_zenbleed(void)
 {
 	const struct cpu_signature *sig = &this_cpu(cpu_sig);
-	unsigned int good_rev, chickenbit = (1 << 9);
-	uint64_t val, old_val;
+	unsigned int good_rev;
+	uint64_t val, old_val, chickenbit = (1 << 9);
 
 	/*
 	 * If we're virtualised, we can't do family/model checks safely, and