AP_HAL: use init() method without arguments

Override the init() method from parent class that doesn't have a
parameter since it's not used here.

libraries/AP_HAL/examples/Storage/Storage.cpp

@@ -14,7 +14,7 @@ void setup(void)
       init Storage API
      */
     hal.console->printf("Starting AP_HAL::Storage test\r\n");
-    st->init(NULL);
+    st->init();
 
     /*
       Calculate XOR of the full conent of memory