staging: greybus: gbphy: make gbphy_bus_type const

Now that the driver core can properly handle constant struct bus_type,
move the gbphy_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
Cc: linux-staging@lists.linux.dev
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/2024010508-fossil-glove-c2c6@gregkhSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/gbphy.c

@@ -182,7 +182,7 @@ static void gbphy_dev_remove(struct device *dev)
 	pm_runtime_dont_use_autosuspend(dev);
 }
 
-static struct bus_type gbphy_bus_type = {
+static const struct bus_type gbphy_bus_type = {
 	.name =		"gbphy",
 	.match =	gbphy_dev_match,
 	.probe =	gbphy_dev_probe,