jtag/drivers/ftdi: Use correct command error

Change-Id: I625acdd8a226528de86f113582e31755d679e68e
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/7970
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

src/jtag/drivers/ftdi.c

@@ -837,7 +837,7 @@ COMMAND_HANDLER(ftdi_handle_set_signal_command)
 		/* fallthrough */
 	default:
 		LOG_ERROR("unknown signal level '%s', use 0, 1 or z", CMD_ARGV[1]);
-		return ERROR_COMMAND_SYNTAX_ERROR;
+		return ERROR_COMMAND_ARGUMENT_INVALID;
 	}
 
 	return mpsse_flush(mpsse_ctx);