AP_HAL: make available_memory() uint32_t

libraries/AP_HAL/Util.h

@@ -52,10 +52,9 @@ public:
     virtual bool get_system_id(char buf[40]) { return false; }
 
     /**
-       how much free memory do we have in bytes. If more than 0xFFFF
-       then return 0xFFFF. If unknown return 4096
+       how much free memory do we have in bytes. If unknown return 4096
      */
-    virtual uint16_t available_memory(void) { return 4096; }
+    virtual uint32_t available_memory(void) { return 4096; }
 
     /**
        return commandline arguments, if available