xen/arm: psci: Populate arm_smccc_res on PSCI_FEATURES call

Commit 0bc6a68da5 "xen/arm: Replace call_smc with arm_smccc_smc"
mistakenly forgot to populate arm_smccc_res. So a garbage value was
used as return value.

Coverity-ID: 1476827
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>

xen/arch/arm/psci.c

@@ -87,7 +87,7 @@ static int __init psci_features(uint32_t psci_func_id)
     if ( psci_ver < PSCI_VERSION(1, 0) )
         return PSCI_NOT_SUPPORTED;
 
-    arm_smccc_smc(PSCI_1_0_FN32_PSCI_FEATURES, psci_func_id, NULL);
+    arm_smccc_smc(PSCI_1_0_FN32_PSCI_FEATURES, psci_func_id, &res);
 
     return PSCI_RET(res);
 }