Bluetooth: controller: Fix tIFS calc for Coded PHY

Always use S8 Rx Chain Delay instead of the actual Rx-ed
packet coding. I believe, as the packet always start with
S8, hence S8 timings when used the tIFS is near correct
value.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>