mavlink: always forward messages to/from USB

Previously, it was not possible to enable forwarding of messages to/from
teh USB instance because it does not have a param for it, like the
serial instances have.

With this commit, we change the default to always set forwarding on for
the USB instance as that is likely desired.

Signed-off-by: Julian Oes <julian@oes.ch>

src/modules/mavlink/mavlink_main.cpp

@@ -2119,6 +2119,9 @@ Mavlink::task_main(int argc, char *argv[])
 		_ftp_on = true;
 		_is_usb_uart = true;
 
+		// Always forward messages to/from the USB instance.
+		_forwarding_on = true;
+
 		set_telemetry_status_type(telemetry_status_s::LINK_TYPE_USB);
 	}