iommu/sysfs: constify the class struct

All functions that take the class address as argument expect a const
pointer so we can make the iommu class constant.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20241018121725.61128-1-brgl@bgdev.pl


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