iommu: make the pointer to struct bus_type constant

iommu: make the pointer to struct bus_type constant

A number of iommu functions take a struct bus_type * and never modify
the data passed in, so make them all const * as that is what the driver
core is expecting to have passed into as well.

This is a step toward making all struct bus_type pointers constant in
the kernel.

Cc: Will Deacon <will@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: iommu@lists.linux.dev
Acked-by: Joerg Roedel <jroedel@suse.de>
Link: https://lore.kernel.org/r/20230313182918.1312597-34-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>