modules: hal_nordic: 802.15.4 make possible to disable CSMA-CA Currently if an attempt to disable CSMA-CA in by setting an appropriate flag in another (for example application) CmakeLists.txt it caused lots of warnings. This fix allows higher level CMakeLists.txt to disable CSMA-CA without warnings. Signed-off-by: Artur Hadasz <artur.hadasz@nordicsemi.no>