moxtet: mark moxtet_bus_type as const

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

Cc: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/2023121939-written-guru-db83@gregkh


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