Bluetooth: controller: Fix flow control packet drop

The main purpose of recv_thread is to process incoming events from the
radio and also any buffered items waiting to be dispatched to the Host
and that are pending because of lack of Host buffers.
When an iteration of the recv_thread obtains a element from the radio it
needs to process it immediately, either sending it straight away to the
Host or appending it to the queue. This was not the case before this
patch, where the concurrency of a buffered packet with one coming from
the radio would cause the latter to be "dropped", causing missing
packets.

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