greybus: Fix missing gb_protocol_exit() on gb_exit()

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>

drivers/staging/greybus/core.c

@@ -302,6 +302,7 @@ static int __init gb_init(void)
 
 static void __exit gb_exit(void)
 {
+	gb_protocol_exit();
 	gb_operation_exit();
 	gb_gbuf_exit();
 	gb_ap_exit();