mmc: core: make mmc_bus_type const

mmc: core: make mmc_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the mmc_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-v1-2-ad054dce8dc3@marliere.net
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>