iucv: make iucv_bus const

Now that the driver core can properly handle constant struct bus_type,
move the iucv_bus variable to be a constant structure as well, placing
it into read-only memory which can not be modified at runtime.

Cc: Wenjia Zhang <wenjia@linux.ibm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-s390@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>