Bluetooth: Controller: Single recv thread for HCI-only builds

Updated implementation to use single receive thread to
enqueue HCI ISO data, ACL data and events towards Host when
building HCI-Only samples/applications, i.e. when building
hci_uart, hci_spi or hci_ipc samples (CONFIG_BT_HCI_RAW=y).

This implementation will serialize HCI events and data as
they occur corresponding to on-air timelines of their
occurrences which is how they are generated by the Link
Layer.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>