bus: mhi: host: make mhi_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the mhi_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: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20240823031129.49010-1-kunwu.chan@linux.dev


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>