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/target/xscale.c

@@ -3878,6 +3878,7 @@ static const struct command_registration xscale_command_handlers[] = {
 		.name = "xscale",
 		.mode = COMMAND_ANY,
 		.help = "xscale command group",
+		.usage = "",
 		.chain = xscale_any_command_handlers,
 	},
 	COMMAND_REGISTRATION_DONE