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/flash/mflash.c

@@ -1386,6 +1386,7 @@ static const struct command_registration mflash_config_command_handlers[] = {
 		.mode = COMMAND_CONFIG,
 		.handler = handle_mflash_init_command,
 		.help = "initialize mflash devices",
+		.usage = ""
 	},
 	COMMAND_REGISTRATION_DONE
 };
@@ -1394,6 +1395,7 @@ static const struct command_registration mflash_command_handler[] = {
 		.name = "mflash",
 		.mode = COMMAND_ANY,
 		.help = "mflash command group",
+		.usage = "",
 		.chain = mflash_config_command_handlers,
 	},
 	COMMAND_REGISTRATION_DONE