iio: core: make iio_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the iio_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>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240208-bus_cleanup-iio-v1-1-4a167c3b5fb3@marliere.netSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/industrialio-core.c

@@ -42,7 +42,7 @@ static DEFINE_IDA(iio_ida);
 static dev_t iio_devt;
 
 #define IIO_DEV_MAX 256
-struct bus_type iio_bus_type = {
+const struct bus_type iio_bus_type = {
 	.name = "iio",
 };
 EXPORT_SYMBOL(iio_bus_type);

include/linux/iio/iio.h

@@ -669,7 +669,7 @@ DEFINE_GUARD_COND(iio_claim_direct, _try, ({
 int iio_device_claim_buffer_mode(struct iio_dev *indio_dev);
 void iio_device_release_buffer_mode(struct iio_dev *indio_dev);
 
-extern struct bus_type iio_bus_type;
+extern const struct bus_type iio_bus_type;
 
 /**
  * iio_device_put() - reference counted deallocation of struct device