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.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>