driver model: constify attribute groups

Let attribute group vectors be declared "const".  We'd
like to let most attribute metadata live in read-only
sections... this is a start.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/include/linux/transport_class.h b/include/linux/transport_class.h
index eaec1ea..9ae8da3 100644
--- a/include/linux/transport_class.h
+++ b/include/linux/transport_class.h
@@ -55,7 +55,7 @@
 
 struct transport_container {
 	struct attribute_container ac;
-	struct attribute_group *statistics;
+	const struct attribute_group *statistics;
 };
 
 #define attribute_container_to_transport_container(x) \