RC RSSI I/O FMU Not always displayed

[ryanjAA]

On Main when RC RSSI is plugged in it doesn't always get picked up and displayed. Multiple reboots were needed to get it picked up.

Tested a known setup on a Pixhawk 4

@dagar explained that it is essentially a race condition between the i/o and fmu to pick it up and to possibly skip the rc_input running on FMU side.

Ideally it should be param that can be explicit. Attached is indicative of the issue: