Set empty usage field for commands that do not need parameters

The missing field causes runtime debug message
	BUG: command '%s' does not have the '.usage' field filled out

While there, fix some minor typo in the help messages:
	s/deasert/deassert/
	s/Deasert/Deassert/

Change-Id: If3dd18265cda103ca0d05609f67f4ca58e7cbb27
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5024
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/server/gdb_server.c

@@ -3699,6 +3699,7 @@ static const struct command_registration gdb_command_handlers[] = {
 		.handler = handle_gdb_save_tdesc_command,
 		.mode = COMMAND_EXEC,
 		.help = "Save the target description file",
+		.usage = "",
 	},
 	COMMAND_REGISTRATION_DONE
 };