Bluetooth: Controller: Fix conn timeout due to delayed conn upd ind

Unlike legacy Zephyr Controller's double headed Tx queue
linked list, the new split Zephyr Controller's new LLCP does
not place a control PDU at the head of the Tx queue causing
control PDUs to be delayed on air.

Pause the data enqueue until pending PDUs are transmitted
across before enqueuing LE Connection Update Ind PDU, and
thereafter resume other PDUs being enqueued again.

There can still be connection timeout if enqueued PDUs are
not being transmitted/acked during repeated retransmissions.

Updated bap_unicast_audio_acl_disconnect test to not use
random seed, to be similar to unicast_client test; to get
it to pass without overlapping state/roles casing connection
failed to be established after the changes in this PR.

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