cmd: add missing usage vars

we should have caught them all - hopefully.

Change-Id: I35435317fccaf5ad0216244d69f76db6857bb582
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/381
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/server/telnet_server.c

@@ -611,6 +611,7 @@ static const struct command_registration telnet_command_handlers[] = {
 		.name = "exit",
 		.handler = handle_exit_command,
 		.mode = COMMAND_EXEC,
+		.usage = "",
 		.help = "exit telnet session",
 	},
 	{