fsi: make fsi_bus_type constant

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

Cc: Ninad Palsule <ninad@linux.ibm.com>
Cc: linux-fsi@lists.ozlabs.org
Reviewed-by: Eddie James <eajames@linux.ibm.com>
Link: https://lore.kernel.org/r/2025070100-overblown-busily-a04b@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>