drivers/iommu: Allow IOMMU bus ops to be unregistered

'bus_set_iommu()' allows IOMMU drivers to register their ops for a given
bus type. Unfortunately, it then doesn't allow them to be removed, which
is necessary for modular drivers to shutdown cleanly so that they can be
reloaded later on.

Allow 'bus_set_iommu()' to take a NULL 'ops' argument, which clear the
ops pointer for the selected bus_type.

Signed-off-by: Will Deacon <will@kernel.org>
Tested-by: John Garry <john.garry@huawei.com> # smmu v3
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>