staging: fieldbus: make fieldbus_class constant

Since commit 43a7206b ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the fieldbus_class structure to be declared at build time
placing it into read-only memory, instead of having to be dynamically
allocated at boot time.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240301-class_cleanup-staging-v1-1-34db2a5b0eb0@marliere.netSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/fieldbus/dev_core.c

@@ -152,7 +152,7 @@ static const struct attribute_group fieldbus_group = {
 };
 __ATTRIBUTE_GROUPS(fieldbus);
 
-static struct class fieldbus_class = {
+static const struct class fieldbus_class = {
 	.name =		"fieldbus_dev",
 	.dev_groups =	fieldbus_groups,
 };