i2c: make i2c_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the i2c_bus_type variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Note, the sound/soc/rockchip/rk3399_gru_sound.c also needed tweaking as
it decided to save off a pointer to a bus type for internal stuff, and
it was using the i2c_bus_type as well.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

drivers/i2c/i2c-core-base.c

@@ -692,7 +692,7 @@ static struct attribute *i2c_dev_attrs[] = {
 };
 ATTRIBUTE_GROUPS(i2c_dev);
 
-struct bus_type i2c_bus_type = {
+const struct bus_type i2c_bus_type = {
 	.name		= "i2c",
 	.match		= i2c_device_match,
 	.probe		= i2c_device_probe,

include/linux/i2c.h

@@ -23,7 +23,7 @@
 #include <linux/swab.h>		/* for swab16 */
 #include <uapi/linux/i2c.h>
 
-extern struct bus_type i2c_bus_type;
+extern const struct bus_type i2c_bus_type;
 extern struct device_type i2c_adapter_type;
 extern struct device_type i2c_client_type;
 

sound/soc/rockchip/rk3399_gru_sound.c

@@ -446,7 +446,7 @@ static const struct rockchip_sound_route rockchip_routes[] = {
 
 struct dailink_match_data {
 	const char *compatible;
-	struct bus_type *bus_type;
+	const struct bus_type *bus_type;
 };
 
 static const struct dailink_match_data dailink_match[] = {