gpu: host1x: Make host1x_context_device_bus_type constant

Since commit d492cc2573a0 ("driver core: device.h: make struct
bus_type a const *"), the driver core can properly handle constant
struct bus_type, move the host1x_context_device_bus_type variable
to be a constant structure as well, placing it into read-only memory
which can not be modified at runtime.

Cc: Greg Kroah-Hartman <[e-mail]>
Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Kunwu Chan <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/host1x/context_bus.c

···
 #include <linux/device.h>
 #include <linux/of.h>
 
-struct bus_type host1x_context_device_bus_type = {
+const struct bus_type host1x_context_device_bus_type = {
 	.name = "host1x-context",
 };
 EXPORT_SYMBOL_GPL(host1x_context_device_bus_type);

include/linux/host1x_context_bus.h

···
 #include <linux/device.h>
 
 #ifdef CONFIG_TEGRA_HOST1X_CONTEXT_BUS
-extern struct bus_type host1x_context_device_bus_type;
+extern const struct bus_type host1x_context_device_bus_type;
 #endif
 
 #endif