gpio: gpiolib: make gpio_bus_type const

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

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/gpiolib.c

@@ -63,7 +63,7 @@ static int gpio_bus_match(struct device *dev, struct device_driver *drv)
 	return 1;
 }
 
-static struct bus_type gpio_bus_type = {
+static const struct bus_type gpio_bus_type = {
 	.name = "gpio",
 	.match = gpio_bus_match,
 };