hw/acpi-build: Make next_base easy to follow

It may be hard to read the assignment statement of "next_base", so

S/next_base += (1ULL << 32) - pcms->below_4g_mem_size;
 /next_base = mem_base + mem_len;

... for readability.

No functionality change.
Signed-off-by: NDou Liyang <douly.fnst@cn.fujitsu.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/i386/acpi-build.c

@@ -2394,7 +2394,7 @@ build_srat(GArray *table_data, BIOSLinker *linker, MachineState *machine)
             }
             mem_base = 1ULL << 32;
             mem_len = next_base - pcms->below_4g_mem_size;
-            next_base += (1ULL << 32) - pcms->below_4g_mem_size;
+            next_base = mem_base + mem_len;
         }
         numamem = acpi_data_push(table_data, sizeof *numamem);
         build_srat_memory(numamem, mem_base, mem_len, i - 1,