ACPI: dock: use NULL for pointer

Use NULL instead of 0 for pointers:
drivers/acpi/dock.c:677:75: warning: Using plain integer as NULL pointer
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Len Brown <len.brown@intel.com>

drivers/acpi/dock.c

@@ -29,6 +29,7 @@
 #include <linux/notifier.h>
 #include <linux/platform_device.h>
 #include <linux/jiffies.h>
+#include <linux/stddef.h>
 #include <acpi/acpi_bus.h>
 #include <acpi/acpi_drivers.h>
 
@@ -674,7 +675,7 @@ static ssize_t show_dock_uid(struct device *dev,
 			     struct device_attribute *attr, char *buf)
 {
 	unsigned long lbuf;
-	acpi_status status = acpi_evaluate_integer(dock_station->handle, "_UID", 0, &lbuf);
+	acpi_status status = acpi_evaluate_integer(dock_station->handle, "_UID", NULL, &lbuf);
 	if(ACPI_FAILURE(status)) {
 	    return 0;
 	}