[Bug] PPM and SBUS input not working with SYS_USE_IO = 0 on Pixhawks and Cubes

[julianoes]

Describe the bug

PPM and SBUS is not working when SYS_USE_IO is set to 0 on v1.14 and main.

v1.13 branch, SYS_USE_IO=1: works
v1.13 branch, SYS_USE_IO=0: works

v1.14 branch: SYS_USE_IO=1: works
v1.14 branch: SYS_USE_IO=0: broken

main branch, SYS_USE_IO=1: works
main branch, SYS_USE_IO=0: broken

To Reproduce

Connect SBUS or PPM Rx to a Pixhawk 6X, 6C, or Cube Orange and set SYS_USE_IO to 0: IO PWM disabled (RC only).

Expected behavior

When the param explicitly says "RC only" I expect RC to work.

Screenshot / Media

No response

Flight Log

No log required

Software Version

v1.14 and main are broken

Flight controller

Pixhawk 6X, 6C, Cube Orange, etc.

Vehicle type

None

How are the different components wired up (including port information)

No response

Additional context

Reported by @reedev.

And it came up in https://discuss.px4.io/t/detected-0-channels-to-operate-px4-you-need-at-least-5-channels-pixhawk-6c-herelink-v1-1-cu/34724.

Related to #21388.

[julianoes]

@gnaneswar484 I've deleted your comment which had nothing to do with this issue.