helper: command: return correct error on command 'echo'

In case of incorrect syntax, return ERROR_COMMAND_SYNTAX_ERROR so
the command framework will print the usage string.

Change-Id: I348debc77f470551d54fa77b4da780a48ff539c0
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8802
Tested-by: jenkins

src/helper/command.c

@@ -673,7 +673,7 @@ COMMAND_HANDLER(handle_echo)
 	}
 
 	if (CMD_ARGC != 1)
-		return ERROR_FAIL;
+		return ERROR_COMMAND_SYNTAX_ERROR;
 
 	LOG_USER("%s", CMD_ARGV[0]);
 	return ERROR_OK;