Bluetooth: controller: Move call to k_sem_give() out of the ISR

The execution time of k_sem_give() increased slightly recently, and
since it was being called from the radio ISR this had an impact in the
deadlines not being met.
This change moves the actual call to k_sem_give() to a lower-priority
job and thus out of the ISR to avoid its execution time affecting the
ISR's timing.

Change-id: I76c82df895c6daaffef52786b0c900ee15acb0aa
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>