porthardware.h file update for AVR Mega0 and Dx (#212)

* Added guard for ioavr.h include in AVR Dx porthardware.h file.
* Added guard for ioavr.h include in AVR Mega0 porthardware.h file.
diff --git a/portable/IAR/AVR_AVRDx/porthardware.h b/portable/IAR/AVR_AVRDx/porthardware.h
index 2d17b42..dbbcd05 100644
--- a/portable/IAR/AVR_AVRDx/porthardware.h
+++ b/portable/IAR/AVR_AVRDx/porthardware.h
@@ -1,7 +1,9 @@
 #ifndef PORTHARDWARE_H
 #define PORTHARDWARE_H
 
-#include <ioavr.h>
+#ifndef __IAR_SYSTEMS_ASM__
+    #include <ioavr.h>
+#endif
 #include "FreeRTOSConfig.h"
 
 /*-----------------------------------------------------------*/
diff --git a/portable/IAR/AVR_Mega0/porthardware.h b/portable/IAR/AVR_Mega0/porthardware.h
index 4188ae2..793d589 100644
--- a/portable/IAR/AVR_Mega0/porthardware.h
+++ b/portable/IAR/AVR_Mega0/porthardware.h
@@ -1,7 +1,9 @@
 #ifndef PORTHARDWARE_H
 #define PORTHARDWARE_H
 
-#include <ioavr.h>
+#ifndef __IAR_SYSTEMS_ASM__
+    #include <ioavr.h>
+#endif
 #include "FreeRTOSConfig.h"
 
 /*-----------------------------------------------------------*/