move overrun detection and reset into the abstract superclass for ACIAs

src/main/java/com/loomcom/symon/devices/Acia.java

@@ -108,11 +108,16 @@ public abstract class Acia extends Device {
 
     public synchronized int rxRead() {
         lastRxRead = System.nanoTime();
+        overrun = false;
         rxFull = false;
         return rxChar;
     }
 
     public synchronized void rxWrite(int data) {
+        if(rxFull) {
+            overrun = true;
+        }
+        
         rxFull = true;
 
         if (receiveIrqEnabled) {

src/main/java/com/loomcom/symon/devices/Acia6850.java

@@ -87,21 +87,6 @@ public class Acia6850 extends Acia {
         }
     }
     
-    @Override
-    public synchronized int rxRead() {
-        overrun = false;
-        return super.rxRead();
-    }
-    
-    @Override
-    public synchronized void rxWrite(int data) {
-        if(rxFull) {
-            overrun = true;
-        }
-        super.rxWrite(data);
-    }
-
-
     private void setCommandRegister(int data) {
         commandRegister = data;