iommu: Use enum iommu_hw_info_type for type in hw_info op

iommu: Use enum iommu_hw_info_type for type in hw_info op

Replace u32 to make it clear. No functional changes.

Also simplify the kdoc since the type itself is clear enough.

Link: https://patch.msgid.link/r/651c50dee8ab900f691202ef0204cd5a43fdd6a2.1752126748.git.nicolinc@nvidia.com
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>