incoming RC signals offset by -5 usec across whole range with F.PORT

## Bug report

**Issue details**
In mission planner if I connect to the flight controller and go to the “radio calibration” page, it reports the channels as centered at 1495usec for both sticks and switches. The radio reports it is sending 1500usec. The endpoints are also offset by -5 usec, so it appears to be a constant offset of -5 usec across the whole range.

My transmitter is a TX16s sending via the internal multi-protocol module to an FrSky RXSR receiver, which is talking F.port to a Matek H743 wing v2. So it should be a digital signal all along.

If it is truly a constant offset then its no big deal, but the fact that there's any offset at all makes me think there's a software bug, and that might potentially have other impacts.

I flashed the same hardware with Betaflight and on the receiver page the channel outputs agree with the transmitter channel monitor.

**Version**
4.1.7

**Platform**
[ ] All
[ ] AntennaTracker
[ ] Copter
[x] Plane
[ ] Rover
[ ] Submarine

**Airframe type**
bench testing

**Hardware type**
My transmitter is a TX16s sending via the multi-protocol module to an FrSky RXSR receiver, which is talking F.port to a Matek H743 wing v2.

**Logs**
N/A

Start at Mission Planner’s radio calibration page with the listed TX16s, FrSky RXSR, Matek H743 wing v2, and Plane setup, then compare its channel values with the transmitter monitor and Betaflight’s receiver page. Investigate the F.Port signal path for the reported constant -5 usec offset; done means determining whether the offset is expected or correcting it so the displayed channels and endpoints agree.

Written by the indexing model from the issue text.