Bluetooth: controller: split: Fix data PDU leak during ctrl PDU defer

Fix a bug where in tx data PDU enqueued, while a ctrl PDU is
deferred due to Encryption setup being in progress, is
leaked causing HCI Tx Buffer Overflow crash.

Relates to commit 671ccc4b0ee6 ("Bluetooth: controller: Fix
data PDU leak during ctrl PDU defer").

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