iommu: constify pointer to bus_type

Make pointer to bus_type a pointer to const for code safety.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20240216144027.185959-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>