[t24_fix]: rxts_calibrator: fix generating second sample of rising edge from falling edge

Previous one was not working when:
* tR and tF were both inside (0; 4000) and tR < tF
* tR and tF were both inside (4000; 8000) and tF < tR

in those cases it was calculating ttrans around falling edge instead of rising
edge. This commit fixes it.