Bluetooth: Controller: Unify handling of async events

This changes the way that aysnchronous events are handled
in the BLE controller's HCI layer. It performs all common
operations in a single place to avoid code repetition of the
same memory accesses over and over.

Jira: ZEP-726

Change-Id: Id9e8395944e8800f32dd9749da2b4f5c901c8682
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>