Bluetooth: Controller: Use defines for TRX and IFS Radio End Events

Use defines for Transmit/Receive and Inter-frame spacing
related Radio End events. This will ease some spaghetti of
conditional compilations for single timer and direction
finding support.

Direction Finding is not yet supported when using single
timer software tIFS switching.

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