trx: factor out FN timer processing into common helper
Extract core FN timer handling logic into trx_fn_timer_process()
to decouple it from timerfd-specific code.
This allows reusing the same scheduling logic for alternative
timer backends (e.g. WebSDR) while keeping timerfd integration
as a thin wrapper.
No functional changes intended.
Change-Id: Ia28cce57afb3dc97c8655e855ebd5ace7e096648
