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

drivers/spi/spi.c

@@ -459,7 +459,7 @@ static void spi_shutdown(struct device *dev)
 	}
 }
 
-struct bus_type spi_bus_type = {
+const struct bus_type spi_bus_type = {
 	.name		= "spi",
 	.dev_groups	= spi_dev_groups,
 	.match		= spi_match_device,

include/linux/spi/spi.h

@@ -36,7 +36,7 @@ struct spi_message;
  * INTERFACES between SPI master-side drivers and SPI slave protocol handlers,
  * and SPI infrastructure.
  */
-extern struct bus_type spi_bus_type;
+extern const struct bus_type spi_bus_type;
 
 /**
  * struct spi_statistics - statistics for spi transfers