ACPI: CPPC: Fix uninitialized ref variable in cppc_get_perf_caps()

Commit 8505bfb ("ACPI: CPPC: Move reference performance
to capabilities") introduced a logical error when retrieving
the reference performance.

On platforms lacking the reference performance register, the fallback
logic leaves the local 'ref' variable uninitialized (0). This causes
the subsequent sanity check to incorrectly return -EFAULT, breaking
amd_pstate initialization.

Fix this by assigning 'ref = nom' in the fallback path.

Fixes: 8505bfb ("ACPI: CPPC: Move reference performance to capabilities")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20260310003026.GA2639793@ax162/
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
[ rjw: Subject tweak ]
Link: https://patch.msgid.link/20260311071334.1494960-1-zhangpengjie2@huawei.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: Pengjie Zhang

drivers/acpi/cppc_acpi.c

@@ -1407,12 +1407,11 @@ int cppc_get_perf_caps(int cpunum, struct cppc_perf_caps *perf_caps)
 	 * If reference perf register is not supported then we should
 	 * use the nominal perf value
 	 */
-	if (CPC_SUPPORTED(reference_reg)) {
+	if (CPC_SUPPORTED(reference_reg))
 		cpc_read(cpunum, reference_reg, &ref);
-		perf_caps->reference_perf = ref;
-	} else {
-		perf_caps->reference_perf = nom;
-	}
+	else
+		ref = nom;
+	perf_caps->reference_perf = ref;
 
 	if (guaranteed_reg->type != ACPI_TYPE_BUFFER  ||
 	    IS_NULL_REG(&guaranteed_reg->cpc_entry.reg)) {