Regression: Betaflight Passthrough Broken in 9.1.0

[robotgoat]

Current Behavior

With the ports and receiver setup properly for an ELRS serial receiver (CRSF) mode selected in receiver tab, flashing new ELRS firmware onto the RX fails via Betaflight Passthrough using ELRS Configurator.

Steps to Reproduce

1. Properly connect an ELRS RX to the FC.
2. Set RX to the UART it is connected to.
3. In Receiver Tab, ensure the RX mode is set to CRSF.
4. Close INAV configurator, restart FC, open ELRS Configurator and attempt to flash new ELRS firmware onto the RX using Betaflight Passthrough.

Expected behavior

ELRS Configurator should be able to detect the RX and flash it successfully.

Suggested solution(s)

There appears to be a major breaking change in serial.c.

Additional context

This problem appears to affect all boards.

• FC Board name and vendor: Matek F405 TE SD
• INAV version string: 9.1.0