greybus: protocol.c: fix a kernel panic caused by __gb_protocol_register

__gb_protocol_register check if the protocol is not already registred,
and then register it. It register in existing->lists but at this point,
existing is always NULL (we exist just before if not).
Use gb_protocols instead.
Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/protocol.c

@@ -70,7 +70,7 @@ int __gb_protocol_register(struct gb_protocol *protocol, struct module *module)
 	 * We need to insert the protocol here, before the existing one
 	 * (or before the head if we searched the whole list)
 	 */
-	list_add_tail(&protocol->links, &existing->links);
+	list_add_tail(&protocol->links, &gb_protocols);
 	spin_unlock_irq(&gb_protocols_lock);
 
 	pr_info("Registered %s protocol.\n", protocol->name);