Fix WAttribute::rollback() for DEV_USHORT (#541) (#542)

The DevUShort WRITE only attribute value was not rolled back correctly to the previous valid value in 
some specific cases (exception thrown in the device server write method associated to this attribute, 
exception thrown by read_attr_hardware() method or exception thrown because it is not allowed to read 
this attribute in the current state).
Now it is!

cppapi/server/w_attribute.cpp

@@ -2644,7 +2644,7 @@ void WAttribute::rollback()
             break;
 
         case Tango::DEV_USHORT :
-            double_val = old_double_val;
+            ushort_val = old_ushort_val;
             break;
 
         case Tango::DEV_UCHAR :