spi: make spi_bus_type const

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

Cc: Mark Brown <broonie@kernel.org>
Cc:  <linux-spi@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://msgid.link/r/2024010549-erasure-swoop-1cc6@gregkh


Signed-off-by: Mark Brown <broonie@kernel.org>