s390/vfio-ap: make matrix_bus const

s390/vfio-ap: make matrix_bus const

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

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Acked-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: "Jason J. Herne" <jjherne@linux.ibm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-s390-v1-6-ac891afc7282@marliere.net
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>