* fix: prevent generic module de-init if protocol is NONE
Fixes#1898
De-initialising the external module while SBUS trainer is used on external module RX leads to the following sequence:
- `extmoduleStop()` is called, thus shutting down the timer DMA
- on certain targets, the timer DMA stream is the same as the external module RX DMA stream
- when this DMA stream is used by the SBUS trainer input, this leads to DMAFifo to return random bytes continuously
- thus driving `processSbusInput()` into an endless loop, which is run in the mixer task
- as the mixer task has the highest priority, it runs forever
- after some time the watchdog timer expires, and causes a reboot
* DMAFifo: check if the stream is enabled
This provides some hardening to avoid the conditions described in previous commit.
