[S390] cio: Use helpers instead of container_of().

- Introduce to_cssdriver.
- Use to_xxx instead of container_of where possible.

Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/css.h

@@ -134,6 +134,8 @@ struct css_driver {
 	void (*shutdown)(struct subchannel *);
 };
 
+#define to_cssdriver(n) container_of(n, struct css_driver, drv)
+
 /*
  * all css_drivers have the css_bus_type
  */