Bluetooth: controller: Use defined keyword in #if cond compiles

Use #if defined(...) constructs while using conditional
compilations of advanced event preparation and advanced
scheduling implementations.

Change-id: I728c76d0e7dbbfa378e8978b726ec404d9e55a72
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>