scsi: core: Make scsi_bus_type const

Since commit d492cc25

 ("driver core: device.h: make struct bus_type a
const *"), the driver core can properly handle constant struct bus_type,
move the scsi_bus_type 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>
Link: https://lore.kernel.org/r/20240212-bus_cleanup-scsi2-v2-1-65004493ff09@marliere.net
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>