The STM32F404 OTG FS has an internal soft pull-up.

The HW also has one too. So we need to overcome it to force a soft disconnect.

N.B The driver set FDMOD, Force device mode and Writing a 1 to this bit
forces the core to device mode irrespective of the OTG_FS_ID input
pin.

This may be moot and should be revisited if only px4_getops is used, but this pr politely documents the reson for the logic.

* Change default port to Serial4 and improve status outputs.

* Reduce stack size.