nvdimm: use *function* directly instead of allocating it again

At the beginning or nvdimm_build_common_dsm(), variable *function* is
already allocated for Arg2.

This patch reuse variable *function* instead of allocating it again.
Signed-off-by: NWei Yang <richardw.yang@linux.intel.com>
Message-Id: <20190227075101.6263-3-richardw.yang@linux.intel.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>

hw/acpi/nvdimm.c

@@ -1086,7 +1086,7 @@ static void nvdimm_build_common_dsm(Aml *dev)
      */
     aml_append(method, aml_store(handle, aml_name(NVDIMM_DSM_HANDLE)));
     aml_append(method, aml_store(aml_arg(1), aml_name(NVDIMM_DSM_REVISION)));
-    aml_append(method, aml_store(aml_arg(2), aml_name(NVDIMM_DSM_FUNCTION)));
+    aml_append(method, aml_store(function, aml_name(NVDIMM_DSM_FUNCTION)));
 
     /*
      * The fourth parameter (Arg3) of _DSM is a package which contains