greybus: make greybus_bus_type const

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

Cc: Alex Elder <elder@kernel.org>
Cc: greybus-dev@lists.linaro.org
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/2024010517-handgun-scoreless-05e7@gregkh


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