mmc: core: make mmc_rpmb_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the mmc_rpmb_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20240203-bus_cleanup-mmc-v1-1-ad054dce8dc3@marliere.netSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/block.c

@@ -144,7 +144,7 @@ struct mmc_blk_data {
 static dev_t mmc_rpmb_devt;
 
 /* Bus type for RPMB character devices */
-static struct bus_type mmc_rpmb_bus_type = {
+static const struct bus_type mmc_rpmb_bus_type = {
 	.name = "mmc_rpmb",
 };