fix(nv14/el18): external module RX inversion

[raphaelcoeffic]

The previous state would set/reset PE15 (unused) instead of PI15 (connected to the inverter).

[richardclli]

I tested this, EL18 v1.0 motherboard, NV14 adapter, RM MPM using 2.8.0 46fc4d9 merged with this PR.
It works properly.

[pfeerick]

iRangeX IR4X+ MPM is working with EL18 (x2) - but it also worked pre this PR. Is there any readily visible external symptom of PI.15 not having been inverted - a particular module that won't work?

NV14 is being completely non-cooperative with MPM (status no telemetry), rolled back to 2.7.1 with no change, so is unrelated to this PR. ELRS, Ghost and XJT modules appear to be working fine though.

[raphaelcoeffic]

Is there any readily visible external symptom

Not that I know of. Theoretically it should affect using ACCESS modules (there is only R9M ACCESS in this format) on the NV14/EL18, where potentially the polarity of the signal could be wrong (depends on default state of the pin / inverter).

I use "theoretically / potentially" because none of this has been tested.

[pfeerick]

Well, in that case, some combination of tries R9M instead of R9M ACCESS, wonders why it doesn't work and flip a coin to guess default state of inverter has come into effect. Plus, you know, TX is not RX (except when it is)... 😁