Bluetooth: controller: Fix regression in central initiated terminate

Fix regression in central initiated terminate introduced in
commit 3a807859 ("Bluetooth: controller: Fix connection
terminate to happen on event done").

The regression caused an additional central connection event
transmitting PDU  after the previous connection event had
received acknowledgment for the terminate_ind PDU from the
peripheral.

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