Bluetooth: Controller: revert to event callback inside radio ISR

In order that host can continue a connection event by
enqueuing packets, the number of completed packets event
needs to be generated as soon it arrives on air. Hence, the
Controller now calls the radio_event_callback on every radio
Rx complete ISR.

The callback executes at the Radio ISR priority, take care
to only do as little as necessary in the callback, say, just
a semaphore give to wake up the hci_driver's thread.

Change-id: If48afd0f1390d450bc1e7ec66f1c9fd45208d9a4
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>