libsel4simple: Use arch_simple to access iospace

libsel4simple/include/simple/simple.h

diff --git a/libsel4simple/include/simple/simple.h b/libsel4simple/include/simple/simple.h
index c73f6a7..eefb885 100644
--- a/libsel4simple/include/simple/simple.h
+++ b/libsel4simple/include/simple/simple.h
@@ -435,12 +435,12 @@
         ZF_LOGE("Simple is NULL");
         return seL4_CapNull;
     }
-    if (!simple->iospace) {
+    if (!simple->arch_simple.iospace) {
         ZF_LOGE("%s not implemented", __FUNCTION__);
         return seL4_CapNull;
     }
 
-    return simple->iospace(simple->data, domainID, deviceID, path);
+    return simple->arch_simple.iospace(simple->data, domainID, deviceID, path);
 }
 #endif